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PREFACE 


This volume comprises the non-mathematical portions of a 
course of lectures, entitled “ Electric Waves and their Application 
to Wireless Telegraphy,” which for several years have been given 
by the author to classes at Harvard University. In giving the 
lectures and in preparing this volume, the design has been: — 
First, to present, in as elementary a form as possible, the course of 
reasoning and experimentation that has led to the conception of 
electric waves; second, to follow this with a discussion of the 
properties of electric waves and electric oscillations; third, to give 
a history of the application of electric waves to wireless telegraphy ; 
and fourth, to elaborate the general principles and methods of 
electric- wave telegraphy in sufficient detail to be of possible use 
to elementary students of electricity and to amateur and pro- 
fessional electricians engaged in operating and constructing wire- 
less telegraphic apparatus. 

The author wishes to express his sincere thanks to Commander 
8. S. Robison of the United States Navy, to Mr. Elliott Woods 
of Washington, and to Chief Inspector D. M. Maliood of the New 
York Navy Yard for their kindness in supplying photographs 
for some of the illustrations. Also, the author is grateful to the 
Editors of the Physical Review for the loan of Plates I and II, and 
to Mr. Greenleaf Whittier Pickard for the privilege of consulting 
his manuscript account of experiments on the effects of daylight 
on transmission. Finally, the author takes great pleasure in 
expressing his gratitude to his friend Mr. George Francis Arnold, 
who has kindly read the proofs and made many valuable sug- 
gestions. 

G. W. PIERCE. 


Harvard University, Cambridge, Mass. 
July , 1910 . 



TABLE OF CONTENTS 


CHAPTER I 

Page 

Introduction 1 

CHAPTER II 

On Theories as to the Nature of Electricity 6 

CHAPTER III 

On the Relation between Electricity and Magnetism 12 

CHAPTER IV 

On the Resemblance of Self-Induction to Mechanical Inertia . 20 

^ CHAPTER V 

On Electrostatic Capacity 23 

CHAPTER VI 

On the Discharge of a Condenser through an Inductance ani> 

Resistance 2S 

CHAPTER VII 

Maxwell’s Theory. Electric Waves. The Electromagnetic 

Theory of Right 30 

CHAPTER VIII 

The Experiments of Hertz 42 

CHAPTER IX 

Experiments on the Identity of Electric Waves and Light 51 

CHAPTER X 

On the Propagation of Electric Waves on Wires 62 

CHAPTER XI 

Wireless Telegraphy before Hertz 75 

vii 



TABLE OF CONTENTS ix 

CHAPTER XXV 

Page 

Directed Wireless Telegraphy . 296 

CHAPTER XXVI 

Wireless Telephony 305 

CHAPTER XXVII 

Some Details op Construction of Wireless Telegraphic Appa- 
ratus 312 

CHAPTER XXVIII 

Conclusion 327 

APPENDIX I 

Elementary Facts about Electricity, and Definitions of Units 329 

APPENDIX II 

Concerning the Calculation of Resistance, Self-Inductance and 

Capacity. . 337 



WIRELESS TELEGRAPHY 


CHAPTER I 
INTRODUCTION 

Almost every one has seen and heard the noisy, brilliant spark 
produced by the discharge of a Leyden jar. The experiment, 
shown in elementary courses in physics, is usually performed as 
follows: The inner and outer coatings of the Leyden jar are 
connected to the terminals of a static electric machine. The 
machine is set in rotation and the jar is charged. After the jar 
has been charged, the electric machine is disconnected, and one 
end of a metallic rod, held by an insulated handle (see Fig. 1), is 




touched against the outer coating of the jar, while the other end 
of the rod is made to approach a knob connected with the inner 
coating. Before the conductor to the inner coating is actually 
touched, a discharge occurs through the metallic rod, producing a 
vivid spark at the gap intervening between the knob and the dis- 
charge rod. As a variation of the experiment, in the place of the 
straight or, slightly curved metallic rod used in the discharge appa- 
ratus of Fig. 1, a coil consisting of a few turns of heavy wire may 

1 



INTRODUCTION 


8 


the Leyden jar always charged in the same direction by the 
electric machine used to charge the jar, the needle was some- 
times found to be magnetized in one direction and sometimes 
in the opposite direction, indicating that the current that pro- 
duced the magnetization of the needle was flowing in the coil in 
the <me case from the outside of the jar towards the inner coat- 
ing,. while in the other case it was flowing from the inner coating 
to the outer coating. This effect could be explained by supposing 
that the current from the Leyden jar was oscillatory, having first 
one direction and then the other, and that the magnetization of 
the needle was reversed at each reversal of the current, the direc- 
tion of the magnetization at the end of the experiment being 
fortuitously determined by the direction last taken by the current. 
Professor Henry's experiment, though not conclusive, gave strong 
evidence of the oscillatory character of the discharge; and the 
opinion that the discharge is 'oscillatory was repeatedly expressed 
and defended by Professor Henry in a number of papers and 
scientific addresses delivered between 1842 and 1850. 

Sir William Thomson's Theoretical Proof of the Oscillatory 
Nature of the Discharge of the Leyden Jar. — In 1853 Sir William 
Thomson, 1 who was afterwards Lord Kelvin, proved by mathe- 
matical reasoning that under certain conditions the discharge 6f a 
Leyden jar occurs in an oscillatory manner. Under certain other 
conditions the discharge is non-oscillatory. In the ease of the 
oscillatory discharge the electricity does not simply flow from one 
coating to the other until the jar is in a condition of electric neu- 
trality, but rushes back and forth between the two coatings a 
great number of times, with a frequency depending on the dimen- 
sions of the jar and the dimensions and form of the coil through 
which the discharge occurs. 

Feddersen's Revolving-Mirror Experiment. — In 1859 Doctor 
Feddersen of the University of Leipzig, by a very beautiful experi- 
ment, proved the correctness of the surmise of Henry and the 
mathematical predictions of Thomson. Feddersen's experiment 
consisted in photographing the spark produced by the discharge 
of the Leyden jar. A photograph similar to that obtained by 
Feddersen is shown in Fig. 3. A sketch of the apparatus used in 
taking the picture is shown in Fig. 4. Instead of employing an 
ordinary camera to take the picture, the light from the spark S, 
produced by the discharge of the jar, was allowed to fall upon 

1 Wm. Thomson: Philosophical Magazine [4], 5, p. 393, 1853. 



INTRODUCTION 


5 


Electric Waves. Maxwell’s Theory. — In a letter to C. H. Cay, 
Esq., dated 5th of January, 1865, James Clerk Maxwell, then Pro- 
fessor of Physics in the University of Edinburgh, wrote: 

“ I have also a paper afloat with an electromagnetic theory of 
light, which till I am convinced to the contrary; I hold to be great 
guns.” 

This paper to which Maxwell referred contained a prediction, 
based on careful mathematical reasoning, that electric oscillations 
in a circuit produce electric waves in surrounding space, that these 
waves travel away with the velocity of light, and that light itself 
is simply a train of electric waves of extremely short wave length. 
This prediction of Maxwell, correlating the phenomena of light 
and electricity, is one of the most beautiful philosophic specula- 
tions in the history of science, and long remained without direct 
experimental confirmation; but now, thanks to the brilliant experi- 
ments of Heinrich Hertz, the existence of electric waves with 
properties intimately related to those of light waves is a well- 
established fact of experience capable of verification in even 
very elementary physical laboratories. 

It is by means of these electric waves that the signals of wireless 
telegraphy and telephony are propagated through space. 

In the succeeding chapters, we shall take up more in detail the 
course of reasoning that led to Thomson’s and Maxwell’s pre- 
dictions, the course of experimenting that led to the proofs of the 
existence of their electric oscillations and electric waves, and the 
development of the very striking methods that have been employed 
in utilizing these electric oscillations and electric waves in the 
transmission of signals. The discussion will introduce some details 
apparently remote from commercial usefulness; but it should he 
home in mind that it has been by means of persistent ami labo- 
rious study of these details that the practical result has been 
attained. 



THEORIES AS TO THE NATURE OF ELECTRICITY 


7 


trifying a body consists in adding to it a quantity of the positive 
fluid or taking from it a quantity of the negative fluid. The state 
of electrification of a body is hence determined by the excess in 
amount of one of the fluids over the other/ In order to account 
for the fact that the appearance of electrification of one sign is 
always accompanied by the appearance of an equal amount of 
electrification of the opposite sign, the two fluids were supposed 
to be uncreatable and -indestructible, so that the accumulation of 
positive electricity in one body is always accompanied by the loss 
of positive electricity in some other body. This is the principal 
property that the electrical fluids were supposed to have in com- 
mon with ordinary material fluids; namely, the property of conserva- 
tism in amount according to which the total amount of electricity 
in a given system can only he changed by the transfer of electricity 
through the boundary of the system . 

The electrical fluids, on the other hand, must possess properties 
that do not belong to the material fluids; for example, portions of 
the positive fluid must be supposed to repel each other, as do also 
portions of the negative fluid, while the two unlike fluids attract 
each other. Another property of the electrical fluids still more at 
variance with the known properties of material fluids is found in 
the fact that if we. add equal quantities of the two electrical fluids 
to the same body, the condition of the body will be unchanged, so 
that according to this theory we must suppose that “ the mixture 
of the two fluids in equal proportions is something so devoid of 
physical properties that its existence has never been detected / 7 1 

THE ONE-FLUID THEORY 

Benjamin Franklin attempted to describe the phenomena of 
electricity in terms of a single fluid. According to his theory, 
one of the fluids, the positive, was retained and called the electric 
fluid , while the other, the negative fluid of the two-fluid theory, 
was replaced by ordinary matter. Quantities of the electric fluid 
were supposed to repel other quantities of the fluid according to 
the law of the inverse square of the distance and to attract matter 
according to the same law. Quantities of matter were supposed 
to repel each other and attract the electric fluid. According to 
Franklin's theory an excess of the electric fluid rendered the body 
positive, while a deficiency rendered it negative. 

1 J. J. Thomson, Electricity and Matter, Charles Scribner’s Sons, 1904. 



THEORIES AS TO THE NATURE OE WLKQtmCLTY Ji 

ciated with them an equal small quantity of electricity or air:inte- 
gral multiple thereof . That is, the charges we meet with are never 
fractional parts of the charge carried by the hydrogen atom; 
whence we may suppose that the latter charge is an elemental 
quantity of electricity. In discussing the evidence afforded by 
Faraday's experiments Helmholtz 1 says that “ if we accept the 
hypothesis that the elementary substances are composed of atoms, 
we cannot avoid the conclusion that electricity, positive as well 
as negative, is divided into definite elementary portions which 
behave like atoms of electricity.*’ 

The study of the conduction of electricity through gases gives 
still stronger evidence of the atomic character of electricity. Gases 
under the action of certain agencies — Roentgen rays, ultra-violet 
light, radium, high electromotive forces, electric spark, etc. — 
become conductive and retain their conductivity long enough to 
permit a study of the mechanism by which the electricity is con- 
ducted. As in the case of the stud\ r of conduction in liquids, we 
are again “ led to the conception of a natural unit or atom of 
electricity of which all charges are integral multiples, just as the 
mass of a quantity of hydrogen is an integral multiple of the mass 
of a hydrogen atom.” 2 

By the study of conduction in gases definite information is 
obtained in regard to the magnitude of this charge. In a series 
of experiments performed chiefly at the Cavendish Laboratory of 
Cambridge University the quantity of electricity in one electrical 
atom is found to be 3.4 X ICC 10 electrostatic c. g. s. units. 3 * This 
quantity obtained from experiments on conduction in gases is the 
same as the quantity of electricity carried by one hydrogen atom 
in the electrolysis of liquids. 

Mass of the Carriers of Electricity. — Also at the Cavendish 
Laboratory evidence as to the. mass of the carriers of electricity 
has been obtained by an experimental determination of the ratio 
of e/m , in which e is the elemental charge and m is the mass of 
matter carrying the charge. The result obtained is that the mass 
of the carrier, when the electricity is negative , is about 1/1700 of the 
mass of the hydrogen atom. This mass is apparently the same 

1 J. J. Thomson, Electricity and Matter, p. 73, Charles Scribner's Sons, 
1904. 

2 J. J. Thomson, Electricity and Matter, p. 83, Charles Scribner’s Sons, 

1904. 

* The electrical units ar*s defined in Appendix I. 



RELATION BETWEEN ELECTRICITY AND MAGNETISM 13 


was employed to produce a continuous flow of electricity in wires. 
This continuous flow of electricity in a wire or other conductor 
is an electric current, and was known to produce heating of the 
conductor through which it flows. 

In 1820 a new impetus was given to a study of electricity and 
magnetism by the discovery by Hans Christian Oersted of Copen- 
hagen that magnetism and electricity are interrelated. This dis- 
covery and some of its consequences is described in the succeeding 
paragraphs. 

On the Production of a Magnetic Field by a Current of Elec- 
tricity. — Oersted's discovery was nothing less than the important 
fact that when a pivoted magnetic needle is placed near a wire 
carrying a current of electricity, the magnetic needle tends to set 
itself at right angles to the wire which carries the electric current. 
If the current is reversed, the direction of the deflection of the mag- 
netic needle is reversed. If the wire carrying the current is moved 
from a position below the needle to a position above the needle, 
t he deriection of the needle is again reversed. 

Oersted's discovery has been utilized in the construction of the 
galvanometer, which is a very dedicate instrument for detecting 
and measuring small electric currents. The principle of the gal- 
vanometer is as follows: A magnetic needle pivoted as in the 
ordinary compass, so as to be free to move 
in a horizontal plane, will, if undisturbed, 
take up a position in the magnetic me- 
ridian of the earth; that is, the needle 
will point approximately north and south, 

( M , Fig. 5). Suppose, now, that a wire 
is passed alternately above and below the 
needle several times so as to form a coil 
(C, Fig. 5 ), with its windings in the plane 
of the magnetic meridian. Let a current y ul 5 cA>il an( j noet u 0 
be passed through the coil, so as to flow of galvanometer, 
north above the needle and south below it; the north current 
above the needle and the south current below it both tend to 
deflect the north-seeking end of the magnetic needle to the west, sq* 
that the effect of the current on the needle is multiplied by tiWai 
combined action of the several turns of the conductor arouiS 
the needle. For a highly sensitive galvanometer, the magnetir 
needle instead of being pivoted is delicately, suspended by a fine 
fiber of spun quartz. 




RELATION BETWEEN ELECTRICITY AND MAGNETISM 13 

was employed to produce a continuous flow of electricity in wires. 
This continuous flow of electricity in a wire or other conductor 
is an electric current, and was known to produce heating of the 
conductor through which it flows. 

In 1820 a new impetus was given to a study of electricity and 
magnetism by the discovery by Hans Christian Oersted of Copen- 
hagen that magnetism and electricity are interrelated. This dis- 
covery and some of its consequences is described in the succeeding 
paragraphs. 

On the Production of a Magnetic Field by a Current of Elec- 
tricity. — Oersted's discovery was nothing less than the important 
fact that when a pivoted magnetic needle is placed near a wire 
carrying a current of electricity, the magnetic needle tends to set 
itself at right angles to the wire which carries the electric current. 
If the current is reversed, the direction of the deflection of the mag- 
netic needle is reversed. If the wire carrying the current is moved 
from a position below the needle to a position above the needle, 
the deflection of the needle is again reversed. 

Oersted’s discovery has been utilized in the construction of the 
galvanometer, which is a very delicate instrument for detecting 
and measuring small electric currents. The principle of the gal- 
vanometer is as follows: A magnetic needle pivoted as in the 
ordinary compass, so as to be free to move 
in a horizontal plane, will, if undisturbed, 
take up a position in the magnetic me- 
ridian of the earth; that is, the needle 
will point approximately north and south, 

(M, Fig. 5). Suppose, now, that a wire 
is passed alternately above and below the 
needle several times so as to form a coil 
(C, Fig. 5 ), with its windings in the plane 
of the magnetic meridian. Let a current p K; 5 an( j notK j[ e 

be passed through the coil, so as to flow <>f galvanometer, 
north above the needle and south below it; the north current 
above the needle and the south current below it both tend to 
deflect the north-seeking end of the magnetic needle to the west, so 
that the effect of the current on the needle is multiplied by the 
combined action of the several turns of the conductor around 
the needle. For a highly sensitive galvanometer, the magnetic 
needle instead of being pivoted is delicately, suspended by a fine 
fiber of spun quartz. 




RELATION BETWEEN ELECTRICITY AND MAGNETISM 15 


solenoid the field of magnetic force is seen to be remarkably like 
that obtained with the bar magnet. 

It may be observed that in the case of each of the coils the lines 



Fuj. 7. Magnetic field about Flu. 8. Magnetic field linking with a coil of 
a straight conductor carry- two turns carrying a current, 

ing an electric current. 

of magnetic force depicted 
by the filings interlink with 
the electric current. 

This conception of a field 
of magnetic force about a 
conductor carrying an elec- 
tric current is of funda- 
mental importance in the 
study of electric waves, in 
which the action in the 
medium rather than the ac- 
tion in the wires is the chief 
factor to be reckoned with. 

So long as the electric 
current in the conductor 
remains steady, the mag- 
netic field remains steady. 

With changes in the elec- 
tric current, the magnetic 
field changes. This changing magnetic field about a conductor 
carrying an oscillatory current will later be shown to be one of 
the components of the electric waves produced at the sending 
station of a wireless telegraph system. 



Fig. 9. Magnetic field produced by a 
solenoid. 



RELATION BETWEEN ELECTRICITY AND MAGNETISM 17 

its own introduction, and in the same direction as that given by 
the introduction of the north pole. 

Another way of obtaining a similar result is to employ two coils 
of wire placed near each other but not electrically connected, as 



Fig. 11. Apparatus for showing electromagnetic induction. 

shown in Fig. 11. One of these coils, S, which we will call the 
secondary, is connected with the galvanometer G, while the 
other, called the ; primary , P, may be connected with the ter- 
minals of a galvanic battery B. No current is shown in the gal- 
vanometer when a constant current is sent through the primary; 
but when the current in the primary is made, broken or reversed, 
transient currents are obtained in the galvanometer. That is to 
say, the current in the primary sets up a magnetic field linking 
with the secondary circuit. While the primary current is steady, 
this field is steady and no effect is obtained in the secondary. 
But variations of the current in the primary cause variations of 
the magnetic field and consequently currents in the secondary. 

The variable currents in the secondary art 1 said to be induced 
by the variable currents in the primary, and the phenomenon is 
referred to as electromagnetic induction. It is in part by action of 
this kind that currents at the receiving station of a wireless tele- 
graph system are produced by the aetion of variable currents at 
the sending station. The extension of the effects of electromag- 
netic induction to the case of two circuits widely separated from 
each other we shall see to be the result of the use of extremely 
rapid electric oscillations at the sending station. 

On Mutual Induction. — Let us examine a little more specifi- 
cally the case of electromagnetic induction described in the gal- 
vanometer experiment cited above. 

This experiment shows that when the current in the primary 
coil is increasing, the current induced in the secondary coil is in 



RELATION BETWEEN ELECTRICITY AND MAGNETISM 19 


tromotive force in the secondary is produced by a variable 
magnetic field from the primary inter linking with the secondary. 
Now, if instead of two coils we have one coiralone carrying a vari- 
able current, the variable current produces a variable magnetic 
field linking with the circuit itself, and in consequence a back 
electromotive force is produced in this coil tending to oppose the 
variation of the current in it. This action of the current on 
itself is called self-induction. The back electromotive force due 
to self-induction in the circuit is connected with the current in the 
circuit by the formula 

E 1 = - Lji, (2) 

in which L x is called the coefficient of self-induction , or, more 
briefly, the self-inductance of the circuit. 7 t is an abbrevia- 
tion for the time rate of change of the current. The subscripts 1 
show that all the quantities refer to the same circuit. 

Consistent with equation (2), the self -inductance of a circuit may 
he defined as the back electromotive force of induction in the circuit 
when the current in the circuit is changing at the rate of one unit 
current per second. 

The numerical value of the self-inductance depends on the geo- 
metrical form of the circuit. In Appendix II formulas are given 
for calculating the self-inductance of some simple forms of circuit. 

This discussion of self-inductance is here introduced in quanti- 
tative terms, because this quantity is of fundamental importance 
in the study of oscillatory currents. I am aware that the semi- 
mathematical form in which the idea is presented may fail to give 
a clear conception of the phenomenon, so I propose to attempt 
in the next chapter to describe self-induction by the aid of cer- 
tain familiar analogies. 



SELF-INDUCTION TO MECHANICAL INERTIA 


21 


The correctness of this belief is evidenced by the fact that with 
a fixed current flowing in a wire the self-induction may be greatly 
increased by bending the wire into the form of a coil. Now mak- 
ing the wire into a coil does not change the amount of electricity 
flowing in the wire, but it does change the strength of the mag- 
netic field about the wire. The inertia of the current, therefore, 
has its existence not primarily in the conductor but in the medium 
surrounding the conductor. 

The Contrast of Self-Induction with Resistance and its 
Resemblance to Inertia. — The self-induction of- a circuit acts 
upon the current in a manner entirely different from the manner 
in which resistance acts. The resistance of a circuit always op- 
posed the flow of the current, and when a current is sent through 
a conductor, some of the energy of the current is used up in over- 
coming the resistance of the conductor ; or, more properly speaking, 
some of the electric energy is converted into heat. This is true 
whether the current is increasing or diminishing or is steady; and 
the heat developed is not again completely available for producing 
electric current, so that a continuous supply of energy is needed 
at the source of the electric current to keep up the current 
against the resistance of the circuit. 

Self-induction, on the other hand, does not change the electrical 
energy into heat. When the current is steady, self-induction has 
no effect. If, however, the current is increasing, some of the 
energy supplied to the system is employed in establishing the 
magnetic field. If now the current is allowed to decrease by an 
equal amount, the energy stored up in the magnetic field is re- 
stored to the conductor and helps to maintain the current. Thus, 
during a cyclic 1 change of the current as much energy may be 
obtained from the magnetic field as was given to it. 

Hence, if we have an oscillatory current in a circuit, none of the 
energy of the current is consumed by the action of the self-induc- 
tion, and the supply of energy at the source is wasted only in 
overcoming the resistance of the circuit . 2 

It is apparent that in respect to the consumption of energy self- 
induction resembles inertia in matter. Energy is required in order 

1 A cyclic change is a change from any value A to any other value B , and 
from B back to A again. 

2 Later we shall see that for some forms of circuit this statement is not 
strictly true, because some of the energy may be radiated as electric waves. 
Also in the case of some media, as iron , in the field of magnetic force, some 
of the energy is converted into heat by hysteresis. 



CHAPTER V 


ON ELECTROSTATIC CAPACITY 

The last two chapters have been devoted to a discussion of' 
electric currents and the magnetic field accompanying such cur- 
rents. In order to arrive at a conception of the nature of electric 
waves it is necessary also to give some attention to the action of 
electric charges at rest. This is the subject of electrostatics. Here 
again we must look to Faraday for the fundamental discoveries. 
In the beginning paragraph of his most important research on this 
subject Faraday says : 1 

“ To those philosophers who pursue the inquiry zealously yet 
cautiously, combining experiment with analogy, suspicious of their 
preconceived notions, paying more respect to fact than to theory, 
not too hasty to generalize, and above all things, willing at every 
step to cross-examine their own opinions, both by reasoning and 
by experiment, no branch of knowledge can afford so fine and ready 
a field for discovery xis this.” 

Influence of Intervening Medium on Electric Attraction. — The 

result obtained by Faraday in the research referred to is that the 
electrostatic repulsion or attraction between two charged bodies 
is influenced by the medium intervening between the charged 
bodies. If, for example, we have two fiat metallic plates placed 
parallel to each other, and wo charge one of the plates positively 
and the other negatively, the electrostatic attraction between the 
two charges on the plates will bo less when the plot os are separated 
by glass than when they are separated by air, provided the plates 
are charged with the same quantity of electricity in the two cases. 
The attraction between the charges on the plates with glass inter- 
vening will be about one-sixth as much as that with the same thick- 
ness of air intervening; so that in order to get the same force 
between the charges on the plates in the two cases we must put 
upon the plates with glass between them six times as much elec- 
tricity as is required with air between. 

1 Faraday: Experimental Researches in Electricity and Magnetism, Vol. I, 
Eleventh Series, Nov., 1837. 


23 



ELECTROSTATIC CAPACITY 


25 


Dielectric Constant. — Returning, now, to the function of the 
dielectric in determining the capacity of a condenser, the term 
dielectric constant of a substance is used to designate the capacity 
of a condenser with the substance as dielectric relative to the 
capacity of the same condenser with empty space as dielectric. 
The dielectric constant of air and all the gases at ordinary pressure 
is approximately unity; this means that the capacity of a con- 
denser with a gas as dielectric is not much changed when the gas 
is pumped away. In the example cited above the dielectric con- 
stant of a particular glass is given as six; that is, the quantity of 
electricity that a condenser will contain under a given electro- 
motive force with this glass as dielectric is six times the quantity 
the condenser will contain under the same electromotive force 
when air is substituted for the glass. A table of dielectric con- 
stants, together with some numerical formulas for calculating the 
capacity of some simple forms of condenser and rules for combina- 
tions of condensers in series and parallel, is given in Appendix II. 

General Facts about Energy and Electromotive Force of 
Charged Condenser. — In order to send a charge of electricity into 
a condenser, energy is required, but the energy is not converted 
into heat, as it is in the case of a current of electricity flowing 
through a resistance; for the energy of the charge may be recovered 
as electric energy when the condenser is allowed to discharge. In 
a cyclic process in which a condenser is charged and discharged 
again, there is no loss of availability of energy in the processes that 
occur in the condenser. And when a condenser charges and dis- 
charges several times in an oscillatory manner, it is necessary to 
supply energy from without only in so far as the electric energy 
is radiated or is converted into heat in flowing through some resist- 
ance in the circuit . 1 

It lias undoubtedly been observed by the reader that in respect 
to the reception of energy from the circuit and the return of the 
same amount of energy to the circuit again the medium of the 
condenser behaves somewhat like the medium of the magnetic 
field. There is, however, one; marked difference . In the case of 
the magnetic field, the opposing electromotive force called into play 
by self-induction is proportional to the rate at which the current is 
changing; while, in the case of the condenser, the electromotive force 
V opposing the flow of electricity into the condenser is proportional 

1 This statement is not always strictly true, because in some forms of con- 
denser a small part of the energy is consumed by hysteresis in the dielectric. 



ELECTROSTATIC CAPACITY 


27 


denser. After this condition is reached, no further current flows. 
This process of charging the condenser is described as gradual 
because time is required for the final condition to be established, 
but this time is usually very short. 

Work Done in Charging Condenser. — During this process of 
charging the condenser, the average e.m.f. of the condenser was 
J E; the work 1 done, which is the charge introduced multiplied by 
the e.m.f. of the condenser, is Q X 4 E\ or, substituting for Q its 
value EC, the work W done in charging the condenser is 

W = hEPC. 

1 See definitions of electrical work, in Appendix I. 



DISCHARGE OF CONDENSE 


29 


released. The column of water will vibrate back 
tube so that its level in the left-hand anil of the tube tHTlliCu ggg 
cessively above and below the position a. During each excursion 
the amplitude of the motion is diminished till the water finally 
comes to rest in its initial position. 

Both of these forms of mechanical vibratory motion are easily 
realized in practice, and both bear a marked resemblance to the 
flow of electricity in the discharge of a condenser through an 
inductance and resistance. 


In order now to understand how a condenser discharge may be 
oscillatory in character, suppose a Leyden jar, or other form of 
electrical condenser, of capacity C to be initially charged, say 
from an electric machine, with a quantity of electricity +Qo on 


one plate and —Qo on the other. 
And suppose that the condenser has 
in series with it a self-inductance L , 
and a spark gap S. (Fig. 14.) At 
first let the spark gap be too wide 
for the spark to pass. Positive 
electricity will be distributed over 
the one coating and one knob of 
the spark gap, and negative elec- 
tricity will be distributed over the 
other coating, the coil L and the 
other knob of the spark gap. 

Let Fo be the difference of po- 
tential between the plates of the 



Fici. 14. Leyden jar, indurlanre 
coil, and spark gap. 


condenser. Before the current starts then 4 will be the same dif- 


ference of potential between the knobs of the spark gap, because 
all parts of a conductor in which no current is flowing arc at the 
same potential. 

Let us suppose, now, that the knobs of the spark gap are made 
to approach each other until the gap is short enough for the poten- 
tial to start a spark (i.e., about 39,000 volts to the centimeter, if 
the terminals of the gap are balls 1 cm. in diameter). When the 
spark vstarts, the resistance of the gap suddenly drops to a very 
small value, in some cases to a small fraction of an ohm, 1 and the 
electric current begins to flow across the gap under the action of 
the high difference of potential between the plates. 


1 We have seen in Chapter II that a spark is one of those agencies that 
render gases conductive. 



DISCHARGE OF CONDENSER 


31 


Non-oscillatory Discharge. — If, on the other hand, R 2 is 
greater than 4 L/C, Thomson showed that the discharge is unidi- 
rectional; that is, no reversal of the sign of the charge takes place. 
We should have an analogous condition of affairs with the elastic 
spring used as an illustration if the bob B (Fig. 12) should be 
submerged in a liquid, provided the liquid should offer sufficient 
resistance to the passage of the bob through it. Evidently the 
amount of resistance required to prevent the oscillation of the bob 
will increase with increase of the inertia of the bob and with 
increase of the stiffness of the spring. The former of these cor- 
responds to L, and the latter to the reciprocal of C, so that the 
fact that L/C will occur in the condition for the oscillation or non- 
oscillation of the electrical system might have been anticipated. 

In the case of the water column, if the connecting tube EF 
between the two vertical cylinders in Fig. 13 is made sufficiently 
small to offer enough friction, the motion of the water wall also 
be non-oscillatory. This is analogous to the case of the non- 
oscillatory discharge of the condenser. 

Mathematical Formulas for the Discharge of the Condenser. — 
Thomson derived the following equations for the current i at any 
time t, where t is measured in seconds from the time when the dis- 
charge begins: 


Case I. Tf R 2 < 4 L/C, 

2 Id, 






l 2 LC 


R 2 C\ 


(3) 


in which F 0 = the initial difference of potential, 

R — the resistance, 

L = the self-inductance, 

C = the capacity, and 

e = 2.718281 . . . (base of natural logarithms). 


This is the case of the oscillatory discharge. 
Case II. If R 2 > 4 L/C, 


i =■ 


Vt 


_t_ 

' t 2 _ 


jl 

Ti 


\! R 2 - 


4 L 

C 


e 


3 


( 4 ) 



discharge of condenser 


33 


it will be evident how the curves are drawn; namely, a table is 
made of current for different values of time, by the aid of formula 
(3), and then for each value of time plotted horizontally the cor- 
responding value of curren t is erected vertically, and through the 
points so obtained a smooth curve is drawn. This process resem- 
bles the method employed by navigators to show the route of a 
ship. Each day, or oftener, an observation of latitude and longi- 
tude is made, and a point is put on the map at the intersection of 
the given latitude and longitude; and through the points thus 
obtained at successive observations a smooth curve is drawn, which 
represents the course of the ship, and from which the position of 



Fi<!. 15. Current from a condemnor of capacity .01 microfarad discharging 
through an inductance of .0001 henry. Initial potential 20,000 voltn. 
Resistance zero. 


the ship at points intermediate between the observations may also 
be approximately obtained. 

Curves Showing Condenser Discharge. — The manner in which 
the discharge of a condenser occurs under different conditions is 
represented graphically in the curves of Figs. 15, 10, 17 and 18. 
In these curves the time in millionths of a second is plotted hori- 
zontally, and the current in amperes is plotted vertically. These 
curves are calculated from the formulas given on page 31. In all 
four oases the eapacity, self-inductance and initial potential 
art' the same; namely, C = 10 ” 8 farads, L — 10 “ 4 henrys, 
Vo = 20,000 volts. The only difference between the conditions 
of the discharge in the four cases is the difference in resistance of 
the circuit through which the discharge occurs. 

In Fig. 15 the resistance is supposed to be zero, and we have 


DISCHARGE OF CONDENSER 


35 


case has also the same capacity, self-inductance and initial voltage 
as the preceding cases, but the current is seen to fise only to about 
75 amperes and then gradually to approach zero. 

If the resistance be made greater than 200 ohms, we have 
Case II, in which the discharge is also non-oscillatory. A curve 
representing this case is not given; the form of such a curve is 
somewhat like that of Fig. 18, with the exception that the curve 
does not rise to so great a value and does not approach zero so 
rapidly as does the curve in Fig. 18. 

The Period of Oscillation. — From equation (3), p. 31, it can 
be shown that the period of a complete oscillation of the current, 
in case the discharge is oscillatory, is 


T = 2r 


2LC 

Vi LC - R 2 C? ’ 


( 6 ) 


in which T is the time of a complete oscillation in seconds; L, C 
and R are measured in the same set of units; e.g., henrys, farads 
and ohms respectively; tt is 3.1416 . . . , the ratio of the circum- 
ference to the diameter of a circle. 

Equation (6) is the exact expression for the period, but in most 
practical cases that occur in the use of electric waves it is found 
that the effect of the resistance is inappreciable in its effect on 
the period; that is, in equation (6), R~C 2 is small in comparison 
with 4 LC, so that the expression for the time of a complete oscil- 
lation simplifies to 

T = 2ir VLC. (7) 

This formula is usually sufficiently accurate. For example, in the 
case plotted in Fig. 16, the period of oscillation calculated by equa- 
tion (7) differs from the exact value, obtained from equation (6), 
by one-fourth of one per cent 

The various formulas given in this chapter were first obtained 
mathematically by Sir William Thomson in 1855. In 1859 Fed- 
dersen demonstrated the oscillatory character of the discharge by a 
revolving mirror photograph of the spark, similar to the photo- 
graph shown in Fig. 3 of Chapter I. Since then all of Thomson’s 
equations have beensubmitted to careful tests and have been found 
to be accurate. 



THE ELECTROMAGNETIC THEORY OF LIGHT 


37 


that when a condenser is charged, the condition of things is not 
completely described by saying that a positive charge is given to 
one plate and a negative charge to the other plate of the condenser. 
Faraday showed that something takes place in the medium between 
the plates, and Maxwell makes the assumption that the action in 
the medium partakes somewhat of the nature of an electric current, 
although the medium is an insulating substance. 

It is difficult to determine just how Maxwell imagined this 
action to take place, and different writers have employed different 
mechanisms in the description of the current that Maxwell sup- 
posed to exist in the insulators. One way of representing his idea 
is to suppose that the insulating medium, whether a solid, liquid, 
or gaseous dielectric, or even empty space, is made up of small 
parts, and to suppose that the electricity in these small parts of 
the insulator may flow freely 
in the small parts but can- 
not flow from one part to 
the next. If we call these 
small parts molecules, we 
may describe the current in 
the insulating medium as 
the act of polarizing the 
molecules. That is, for ex- 
ample, when the left-hand 
plate of the condenser in Fig. 

19 is charged positively, the 
positive electricity added to 
this plate attracts the nega- 
tive electricity and repels the positive electricity of the neigh- 
boring molecules, so that the part of each molecule near the plate 
becomes negative and the distant part becomes positive. Mole- 
cules in this condition are said to be polarized. The layer of 
molecules so polarized acts on the next layer and produces a similar, 
polarization, so that in turn the molecules throughout the medium 
between the plates become polarized. 

It is seen that this general transfer of positive electricity to the 
right and negative electricity to the left in the molecules would have 
an effect similar to an electric current flowing from the positive plate 
to the negative through the insulator. Maxwell called this general 
transfer of electricity in the dielectric a displacement current During 
the charging of the condenser, the displacement current is in the 



Fra. 19. Illustrating displacement current. 




THE ELECTROMAGNETIC THEORY OF LIGHT 


39 


passes between them. If the resistance, is not too large, the current 
that flows will be oscillatory, because the rods have electrostatic 
capacity and self-inductance. The two metallic rods here pictured 
constitute an electric “ oscillator/ 7 

According to Maxwell’s theory, the oscillatory currents in the 
oscillator will be completed by displacement currents in surrounding 
space. A part of this displacement current takes place along the 
black loops in the direction of the arrows from one end of the oscil- 
lator around to the other. The displacement loops are really sec- 
tions of a sheet, such as would be obtained if we rotated the figure 



about the oscillator as an axis. These displacement currents in the 
sheet will reverse their direction when the current in the oscillator 
reverses, and are accompanied by a magnetic field of which a single 
line is shown encircling the displacement sheet. The magnetic field 
produced by the displacement current in the shaded region, being 
oscillatory in character, will induce displacement currents in a portion 
Of the medium farther out from the oscillator, and the latter current 
will lag somewhat 1 behind the former. Thus, a sheet corresponding 
to the shaded region will sustain a displacement current oscillating 
with the period of the oscillator. The unshaded region farther out 
will sustain similar oscillations a little later, so that we have the 
condition of things that exists in a wave motion traveling with a 
finite velocity; namely, a series of disturbances first in one direction, 
then in the opposite direction, taking place all over a closed surface, 
and traveling outward from the source. 




THE ELECTROMAGNETIC THEORY OP LIGHT 


39 


passes between them. If the resistance, is not too large, the current 
that flows will be oscillatory, because the rods have electrostatic 
capacity and self-inductance. The two metallic rods here pictured 
constitute an electric “ oscillator.” 

According to Maxwell’s theory, the oscillatory currents in the 
oscillator will be completed by displacement currents in surrounding 
space. A part of this displacement current takes place along the 
black loops in the direction of the arrows from one end of the oscil- 
lator around to the other. The displacement loops are really sec- 
tions of a sheet, such as would be obtained if we rotated the figure 



Ficj. 20. Displacement current and magnetic force. 

about the oscillator as an axis. These displacement currents in the 
sheet will reverse their direction when tin* current in the oscillator 
reverses, and are accompanied by a magnetic field of which a single 
line is shown encircling the displacement sheet. The magnetic; field 
produced by the displacement current in the shaded region, being 
oscillatory in character, will induce displacement currents in a portion 
Of the medium farther out from the oscillator, and the latter current 
will lag somewhat l>ehind the former. Thus, a sheet correspondirtg 
to the shaded region will sustain a displacement current oscillating 
with the period of the oscillator. The unshaded region farther out 
will sustain similar oscillations a little later, so that we have the 
condition of things that exists in a wave motion traveling with a 
finite velocity; namely, a series of disturbances first in one direction, 
then in the opposite direction, taking place all over a closed surface, 
and traveling outward from the source. 



THE ELECTROMAGNETIC THEORY OF LIGHT 


41 


4. All good conductors are opaque to electric waves, all good insu- 
lators are transparent to electric waves, and semiconductors like 
wood and stone are semitransparent. Metallic surfaces are prac- 
tically perfect reflectors of electric waves. 

The Electromagnetic Theory of Light. — Among these several 
properties of electric waves the properties stated in 1 and 2 are 
identically true of electric waves and light; while the properties enu- 
merated in 3 and 4 have also met with very useful application to 
light as well as to longer electric waves. Thus Maxwell came to 
the conclusion that light waves are electric waves of short wave 
length. This theory is now generally accepted. 

It is interesting to note, on this theory, how light can be produced. 
We have seen how electric waves may be produced by oscillating 
electric currents in a circuit of the form shown in Fig. 20. Now if 
wc suppose the oscillator of Fig. 20 to be made smaller and smaller, 
the capacity and inductance will both be decreased, and the time of 
oscillation is thereby decreased. If then we think of the oscillator 
as possessing atomic dimensions, the period of oscillation approaches 
that of light. It is, however, not necessary to think of an actual 
electric; discharge taking place between the atoms of our atomic 
oscillator, because; the rapid vibratory motion of a single charged 
particle, or electron, back and forth would have the same effect as 
an electric discharge between particles, and would produce electric 
waves of which the period, for a particular size and velocity of the 
vibrating particle, would be the period of light of some particular 
color. 

Let us turn next to the experimental demonstration of the exist- 
ence' of the electrical waves predicted by Maxwell. This did not 
come during Maxwell's lifetime; in fact, twenty-two years elapsed 
between Maxwell's remarkably clear presentation of the theory and 
Hertz’s brilliant confirmation of it. 



THE EXPERIMENTS OF HERTZ 


43 


across the spark gap at S'. The two circuits were then in resonance; 
that is to say, they had the same period of oscillation as determined 



Fiu. 22. Sir Oliver Lodge's resonant Leyden jars. 


by the formula T — 2ir\LC\ The oscillatory current in the dis- 
charge circuit induced an electromotive force in the receiving circuit, 
and when the circuits were in resonance, this induced electromotive 
force was capable of forcing sparks across the gap at *S Y/ , even when 
the two circuits were several meters apart. 

According to Maxwell's theory, the inductive action between the 
two circuits consisted of electric waves sent out from the discharge 
circuit and striking the receiving circuit.; but Lodge was not able to 
demonstrate the existence of these waves. To do this it was neces- 
sary to make the wave length shorter and the radiation freer than 
that produced by Lodge's discharge circuit. 

Hertz's Experiments with Electric Waves in Air. — In order to 
produce shorter waves than those employed by Lodge, Hertz made 
use of a discharge system with smaller capacity and self-inductance. 
One form of Hertz's “ oscillator " is shown in Fig. 23. It consists 
of two flat metallic plates, 40 cm. square, each attached to a rod 30 
cm. long. The two rods were placed in the same line, and were 
provided at their nearer ends with balls separated by a spark gap 
about 7 mm. long. The oscillator was charged from the secondary 
of a Ruhmkorff coil J attached to the rods near the spark gap. The 



THE EXPERIMENTS OF HERTZ 


45 


To demonstrate the existence of the electric waves Hertz made use 
of the phenomenon of interference. The arrangement of apparatus 
is shown in Fig. 25. M is a metallic reflector, consisting of a sheet 
of zinc, 2 meters wide by 4 meters high, from which the waves sent 
out by the oscillator are reflected. The reflected waves superimpose 
upon the direct waves, producing in the region between the oscillator 
and the metallic reflector certain positions where the direct and the 
reflected waves neutralize each other and certain other positions in 
which their effects add. In demonstrating these effects Hertz per- 
formed a number of beautiful experiments. 

In one experiment the plane of the resonator was kept parallel to 
the reflector, with the spark gap at the side, as shown in Fig. 25. 
Then wherever the resonator may be placed along the line SN i, the 
electric force F and F f is the same at the two sides of the resonator. 
But the force F', being applied to a completely metallic part of the 
loop, acts to a greater advantage 1 than the force F , so that sparks 
are produced unless both F and F' are very small. With this orien- 
tation of the resonator, Hertz started with the resonator at Ni close 
to the reflector and moved it gradually away toward the oscillator. 

In the position Ni there wore no sparks in the resonator, showing 
that there is a node of electric force at the reflector. This result is 
consistent with the fact that a large difference of potent i ail cannot 
be set up in the surface of a good conductor. As the resonator is 
moved away from the reflector, sparking begins in the resonator, 
becomes more and more lively, until a maximum is reached at L\. 
This position Lj, is called a loop of electric force. On proceed- 
ing further in the same direction, a second minimum of sparking 
is found at No, and so forth. 

Discussion of this Experiment. — The occurrence of maxima 
and minima in the region between the reflector and the oscillator 
is evidence of the undulatory nature of the disturbance, and the 
distance NiNz> or LiL 2 , is the half wave length. To make this 
proposition clear, reference is made to Fig. 26, which shows several 
drawings of the direct and the reflected wave and the resultant 
obtained by their superposition. The reflecting mirror is repre- 
sented by the heavy vertical line at the right. The undulating 
line, made up of dashes, represents the direct wave, which is 
moving toward the reflector; and the dotted wavy line is the 
reflected wave, moving from the reflector. The heavy line in the 

1 In the same way that plucking a violin string at the middle will produg© 
a greater motion than plucking it near the end. 



THE EXPERIMENTS OF HERTZ 


47 


neutralize each other, while at other points their intensities add. 
At Li, L 2 and L 3 the added intensities give a resultant about 
1.4 times the maximum of either wave alone. 

In (c), t = 2 T / 8, the direct wave has approached the mirror by 
another eighth of a wave length, the reflected wave has receded 



from the mirror by an equal amount, and the two waves exactly 
superpose. The resultant intensity of electric force is still zero at 
N i, N 2 , N 3 and N 4 , while at Li, L 2 and L 3 the intensity is double 
that of either wave separately. 

In a similar manner the remaining drawings (d), (e), (/), (g), (h), 
(i) represent the progress of the direct wave toward the mirror and. 
the recession of the reflected wave from the mirror by successive 
ei gh ths of a wave length. The resultant intensity is always zero 



THE EXPERIMENTS OF HERTZ 


49 


either side of the oscillator are shown the lines along which Max- 
well's displacement currents occur. These lines are called lines 
of electric induction. We have seen in Chapter VII how we can 
imagine the displacement current in the dielectric to complete the 
conduction current in the oscillator. In that case the lines of elec- 
tric induction terminate on a positive and a negative charge at 
their two ends. At the instant represented in the diagram, the 
two halves of the oscillator have opposite charges, and some of 
the lines of electric induction near the oscillator terminate upon the 
charges on the oscillator. But a little farther out from the oscil- 
lator the lines in the diagram are represented as closed upon them- 
selves. This closing of a loop on itself occurs when the positive 
and the negative charges on the oscillator come together as the 
current in the oscillator 
reverses. The closed loops 
represented in the diagram 
have been produced by 
successive oscillations of 
the current on the oscilla- 
tor, and have been liber- 
ated from the oscillator 
and are moving freely 
away. The condition of 
things in the space around 
the oscillator in action 
may be pictured to the 
mind by supposing that 
these closed loops of electric induction move away from the 
oscillator, and as they move they elongate and grow loss intense. 
Their width, however, remains constant, so that if a receiver bo 
placed in any fixed position, say in the equatorial plane, PP , 
the inductive action of the loops, as they successively pass, 
changes continuously from one direction to the other with a 
period equal to that of the oscillator. This train of continuously 
reversing electrostatic induction is one aspect of the electric-wave 
train. 

Another aspect of the electric wave train may be discovered by 
examining the magnetic field about the oscillator. The lines of 
magnetic force about the oscillator are circles in a plafie perpen- 
dicular to the oscillator, and these lines in a non-magnetic medium 
are everywhere perpendicular to the lines of electric induction* so 




CHAPTER IX 


EXPERIMENTS ON THE IDENTITY OF ELECTRIC WAVES 

AND LIGHT 


Hertz's Apparatus for Shorter Electric Waves. — After Hertz had 
succeeded in proving that the action of an electric oscillation spreads 
out as a wave into space, he planned experiments with the object 
of concentrating this action and making it perceptible to greater 
distances, by putting the oscillator in the focal line of a large con- 
cave cylindrical mirror. In order to avoid the disproportion between 
the length of the waves and the dimensions he was able to give to the 



Fia. 28. Hertz’s roe- Fin. 29. Hertz’s cylindrical mirrors. Oscillator 
tilinear oscillator. is at left ; resonator, at right. 


mirror, Hertz made the oscillator smaller, ho that the length of the 
waves was less than one-tenth of those first discovered. 

The form of oscillator used in these experiments in shown in 
Fig. 28. The two halves of the oscillator were cylindrical bodies 
3 cm. in diameter, terminating in spheres 4 cm. in diameter. The 
total length of the oscillator was 26 cm., and the spark gap was 
usually about 3 mm. 

For a receiving circuit, the circle of wire used in the previous 
experiments was replaced by a linear resonator, consisting of two 
straight pieces of wire, each 50 cm. long and 5 mm. in diameter, 
adjusted in a straight line so that their near ends were 5 cm. apart. 

51 



THE IDENTITY OF ELECTRIC WAVES AND LIGHT 53 


the resonator completely, while the two screens in the position B and 
B' did not materially diminish the sparks at the .resonator. If, how- 
ever, the opening between B and B' was made narrower, the sparks 
became weaker, and disappeared when the opening was reduced below 
'a half meter. In experiments of this kind, although the dimensions 
of the screens are measured in meters, these screens are yet not large 
in comparison with the wave length of the waves, and the phenomena 
of diffraction are very marked, so that there is no sharp geometrical 
limit either to the rays or to the shadows. 

Polarization. — Hertz showed that the electric waves produced 
by his linear oscillator are polarized waves. One way employed by 
him for showing this was to start with the focal lines of the two reflec- 
tors parallel, as in Fig. 29, so that there is lively sparking at the 



Fro. 31. Showing polarization by the absence of efforts when the 
oscillator and the resonator are at. right angles to each other. 

resonator, and turn the receiving mirror about the line joining oscil- 
lator and resonator. During this operation the resonator sparks 
become more and more feeble, and when the two focal lines are 
at right angle's, as in Fig. 31, no sparks whatever are obtained at 
the resonator, oven when, the two mirrors are moved up close to 
each other. 

In another method of showing that the electric waves are polarized, 
Hertz made use of a grating of wires. The wires of the grating 
were 1 mm. in diameter and 3 cm. apart, and were mounted in an 
octagonal wood frame 2 meters high and 2 meters long. When 
the grating was interposed between the oscillator and the resonator 
so that the direction of the wires of the grating was perpendicular to 
the oscillator and the resonator, as shown in Position 1, Fig. 32, the 
screen practically did not interfere at all with the sparks at the 
resonator. But if the screen was set up in such a way that its wires 



THE IDENTITY OF ELECTRIC WAVES AND LIGHT 55 


tion of its wires. This component is inclined at 45° to the axis of 
the receiver, and so has a component along the direction of the 
resonator. 

From these experiments it is evident that the interposition of the 
screen stops the waves when the wires of the screen are parallel to 



Fig. 33. Rotation of piano of polarization by a wire grating at 45°. 

the electric component of the waves. It is in this position that the 
electric force would produce currents in the wires. The changing 
magnetic force at right angles to the wires would also produce cur- 
rents in the wires, so that both the components, that is to say, the 
whole electric wave, would be absorbed or reflected. Hertz showed 
that the action was one of reflection rather than of absorption; in this 
the wire screen differs from the action of the tourmaline crystal on 
light, for the extinguished component in that case is absorbed rather 
than reflected. 

Refraction. — Hertz also performed some experiments on the 
refraction of electric waves, employing for the purpose a large prism 



Fig. 34. Showing refraction of electric waves by prism. 

of pitch cast in a wooden box. The base of the prism was an isos- 
celes triangle 1.2 meters on the side, and with a refracting angle of 



THE IDENTITY OF ELECTRIC WAVES AND LIGHT 57 


machine used to charge the oscillator. These terminals are provided 
■with the- spheres A and D, which are separated from the spheres B 
and C of the oscillator by spark gaps in air, so that the oscillator BC 
is without metallic connection with the other parts of the circuit. 
The spheres B and C were fastened with shellac into the truncated 
cones of glass EF and GH, which were supported in an ebonite frame. 
The. lower funnel-shaped glass vessel served to contain the oil. The 
spark length in oil between B and C could be regulated by the screw 
V. The advantage of having the spark between the spheres take 
place in oil instead of in air, as had already been pointed out by MM. 
Sarasin and De la Rive, arises from the fact that it takes a greater 
difference of potential to start a given length of spark and therefore 
gives a more energetic discharge. When the spark is once started, 
the oil is carbonized and becomes conducting, so that the succeed- 
ing oscillations pass with comparatively little damping. Also the oil 
obviates the necessity of repeatedly polishing the terminals, as Hertz 
found he had to do when he attempted to get short waves -with the 
spark in air. Righi found that vaseline oil is especially well adapted 
for use with his oscillator. 

For a receiving apparatus Righi made use of a resonator consisting 
of a strip of silver A B deposited on glass and interrupted by a 
diamond scratch C across the middle of the strip. This provided an 
extremely short spark gap between the two parts of the resonator, as 
shown in Fig. 30. Also the spark across this small gap will occur 
more easily than a spark of equal length in free air. 1 ltighi’s reso- 
nator is thus seen to he an extremely sensitive modification of the 
rectilinear resonator used by Hertz. 

Inmost of Righi’s experiments the oscillator and the resonator were 
mounted in cylindrical reflectors. The mounting of the resonator is 
shown in section in Fig. 37. The resonator is at A, and is fastened 
upon a strip of elxmite BC. The observer looks through the con- 
verging lens at //, which servos to magnify the minute sparks between 
the two halves of the resonator. The apparat us could bo list'd quan- 
titatively by observing the angle through which it was necessary to 
turn the resonator and its reflector in the supixirt LM in order to 
extinguish the sparks. The angle of turning was indicated by the 
pointer N moving over a graduated circle OP. 

1 The author has shown that the potential required to start a spark along 
a surface of glass is about .44 of the potential to start a spark of equal length 
in free air. (Pierce: Physical Review, Vol. 2, pi 99, 1894.) 



THE IDENTITY OF ELECTRIC WAVES AND LIGHT 59 


Ignaz Klemencic 1 showed that a thermal junction could be employed 
to detect and measure the waves. Klemeneic’s device, Fig. 38, con- 
sists of two thin sheets of brass MM, 10 cm. broad and 30 cm. long, 
placed 3 cm. apart, and having soldered to them respectively a very 
fine platinum and a very fine platinum-nickel wire, which were 
crossed at k and were thence conveyed off at right angles and soldered 
at their other ends to the leads l, l of a sensitive galvanometer. This 
resonating system was fixed at the focal line of a suitable cylindrical 
metallic reflector. When electric waves, with 
the electric force parallel to MM, fall on this 
receiver, electric oscillations between M and M 
produce heating of the knot k, which is the 


Fid. 3<S. Resonator employing Fro. 39. Oscillator for very 

thermal junction. short electric waves. 

jjoint of contact of two dissimilar metals, and in consequence the 
heat developed gives rise to a thermoelectromotive force at the knot 
and consequently to a current in the galvanometer. By the use of 
this instrument and a Righi oscillator, Klemencic has studied the 
reflection of electric waves from metals and insulators. 

Various investigators have made use of the Klemencic thermal 
junction in quantitative experiments on electric waves. By reducing 
the size of the metal vanes MM, Professor A. D. Cole J has applied 
the apparatus to measurements with waves with a wave length of 
4 cm. Professor Lebedew, 3 employing a slightly different form of 

1 Ignaz Klemericiih Wied. Ann., 45, p. 62, 1892. 

* A. D. Cole: Wied. Ann., 57, p. 290, 1896, and Phys. Review, 7, Nov., 1898. 

3 Peter Lebedew: Wied. Ann., 56, p. 1, 1895, 





THE IDENTITY OF ELECTRIC WAVES AND LIGHT 61 

radiations than these are to the ultra-violet or even to the visible. 
For example, some of the long heat waves, like the Hertzian wares, 
pass readily through vulcanite and other insulators opaque to visible 
light. 

Space is lacking to consider further the experimental evidence in 
favor of Maxwell's proposition that electric waves are of the same 
nature as light waves, and that the light waves are in fact simply 
electric waves of those particular wave lengths that possess the prop- 
erty of being, capable of affecting the retina of the eye. 



THE IDENTITY OF ELECTRIC WAVES AND LIGHT 


radiations than these are to the ultra-violet or even to the visible. 
For example, some of the long heat waves, like the Hertzian waves, 
pass readily through vulcanite and other insulators opaque to visible 
light. 

Space is lacking to consider further the experimental evidence in 
favor of Maxwell’s proposition that electric waves are of the same 
nature as light waves, and that the light waves are jn fact simply 
electric waves of those particular wave lengths that possess the prop- 
erty of being, capable of affecting the retina of the eye. 



THE PROPAGATION OF ELECTRIC WAVES ON WIRES 63 

current, and for this purpose availed themselves of the telegraph 
lines between Paris and Amiens (314 kilometers) and between Paris 
and Rouen (288 km.). Their measurements gave a velocity of 
101,700 km. per second for iron wires, and 172,000 km. per second 
for copper wires. 

In other similar measurements of the apparent velocity of the 
electric current various results have been obtained in practice which 
are much lower than those of Wheatstone, and Fizeau and Gounelle, 
being in some cases 2240 kilometers per second, and in others 4800, 
28,000, 96,000 and so on. What, then, is the explanation of this 
great variability in the experimental results ? 

Theoretical Discussion. — In 1855, in discussing the feasibility of 
an Atlantic cable, Sir William Thomson gave a mathematical treat- 
ment of a case of the propagation of electric disturbances in con- 
ductors. In 1857 Kirchhoff, and in 1876, Heaviside, developed 
extended theoretical treatments of the problem. The results ob- 
tained by these mathematical physicists show that the velocity of 
propagation of electrical disturbances in conductors depends on the 
nature of the disturbance and the a 
relative values of the capacity, 
self-inductance and resistance of 
the conductor. 

If we have two long parallel **“• JXt'mSSv, cl™ 

wires (Fig. 40) as in the case of 

land telegraph and telephone lines, or one wire in an insulating 
sheath submerged in a conducting body, as in the submarine cable, 
three important cases arise in practice. 

Case I. Telegraphy. — If the self-induction of the line is negli- 
gible in comparison with its resistance and we have an electromotive 
force impressed on one end of the line, the current in the conductor 
grows in a manner described as “diffusion.” Fig. 41 gives a set of 
curves 1 showing the difference of potential between the two conduc- 
tors at various positions along the line, at different times after the 
application of the electromotive force. In this case there is no proper 
velocity of the electricity; for at the instant the battery is applied 
some electricity appears all along the line, and the charge at a short 
distance from the origin grows faster than the charge at a greater 
distance. This is approximately the case that occurs in submarine 

1 Redrawn from Professor A. G. Webster’s Electricity and Magnetism; 
Macmillan, 1897. 




THE PROPAGATION OF ELECTRIC WAVES ON WIRES 65 


than for a slow application of the charge. For this reason the prop- 
agation of the disturbance is more accurately represented by the set 
of curves given in Fig. 42. In this diagram it is seen that the 
disturbance has a nearly square wave front, which, according to the 
theory, travels with the velocity of light, while succeeding parts of 
the impulse lag more and more behind the wave front. The square 
wave front itself becomes also more and more attenuated as the 
disturbance progresses along the wires. 

This same condition of things exists to some extent in the case of 
land telegraph lines, and accounts for the indefiniteness of the results 
that have been obtained in the attempt to measure the velocity of 
propagation. If for a particular length of line the apparatus used 
by the experimenter for detecting the wave is sufficiently sensitive 
to respond on the arrival of the wave front, the value obtained for 
the velocity is the velocity of light; while with a greater length of 
line the wave front is too feeble to affect the instrument, which then 
responds to a more intense part of the wave arriving later, and hence 
gives a smaller value for the velocity. 

Case EL Telephoning. — • Suppose, now, that instead of simply ap- 
plying a battery to the line, as in telegraphing, we apply a telephonic 
electromotive force to the parallel wires of Fig. 40 or to the 
submarine cable. This telephonic electromotive force is an alter- 
nating electromotive force. Although the self-induetanco and resist- 
ance of the circuit may be the same as before, the effect of the 
self-induction is larger in the telephonic case, because of the rapidity 
of the alternations of the electromotive force at the source. Under 
this condition Heaviside finds that the different waves generated by 
the sounds of different pitch travel with different velocities, and that 
this results in a distortion of the wave and puts a limit to the dis- 
tance to which the telephone can be used. This distortion is caused 
by the resistance and capacity of the line, and is partially eliminated 
by self-induction. Heaviside says that this “ self-induction is the 
telephonist’s best friend,” for it tends to preserve the sharpness of 
the wave and to eliminate the part of the disturbance lagging behind 
the wave front. Heaviside pointed out that the addition of properly 
distributed self-induction was be?ieficial to prevent distortion in 
telephony; and in actual practice, by adding inductance coils at 
intervals along telephone lines, Professor Pupin has considerably 
increased the distance to which distinct speech may be transmitted. 

In the case of the submarine cable, on account of the relatively 
small value of the self-inductance, submarine telephony is not at 



THE PROPAGATION OF ELECTRIC WAVES ON WIRES 67 


cannot flow past the end of the wire, nor does the electricity con- 
stituting the current merely flow out to the end of the wire and stop 
in a state of equilibrium. Two forces are acting on the current : 

(1) the accumulation of electricity near the end of the wire raises 
the potential of the wire and provides a force opposing the current; 

(2) the slowing down of the current causes change in the magnetic 
field surrounding the wire, and this tends to prevent the cessation of 
the current. These two forces do not act together, — when one is a 
maximum, the other is a minimum. As a result first one and then the 
other of these forces will predominate, so that the charge will first 
be sent into the parts near the end of the wire by the magnetic field 
(self-induction) and will then be sent out again by the electrostatic 
rise of potential (reciprocal of capacity) . The effect of this is that the 
periodically arriving impulses will be sent back again with the same 
period, and we shall have, therefore, a direct and a reflected train of 
waves. The direct and the reflected waves will interfere with each 
other, so as to form a stationary system of waves like that obtained 
in the experiment with waves in air reflected from a sheet of metal 
(Chapter VIII). In this case, however, the end of the wire will be 
a loop of potential; whereas the metal reflector of the waves in air 
is a node of potential. There is also another difference; for in the 
case of the wire, the returning wave will again be reflected at P, and 
a simple stationary wave system can only be realized provided the 
horizontal wire has a proper length, which may be determined by 
experiment. 

Hertz studied the waves produced in the wire, with the aid of his 
circular resonator, shown in the figure. With the resonator in the 
vertical position C, Hertz was able to locate the nodes and loops of 
current in the wire by the absence or presence of sparks at the reso- 
nator. When, however, the resonator was placed in the horizontal 
position B , the effect obtained was due partly to the waves in the 
wires and partly to a linking with the resonator of magnetic lines 
directly from the oscillator. The compound effect obtained in the 
latter case was utilized by Hertz in a study of the interference between 
the waves in the wire and the waves in the air. He came to the con- 
clusion that the wave length, and consequently the velocity of prop- 
agation, was different in the two cases. This was in contradiction 
of Maxwell's theory. 

Later, by the use of a smaller oscillator at A A', he found that 
the difference between the velocities of the waves on wires and in 
air very nearly disappeared. 



THE PROPAGATION OF ELECTRIC WAVES ON WIRES 69 


a resonance method, like that at the present day used in getting 
the wave length in a wireless telegraph antenna. 

The following results were obtained for the velocity of electric 
waves on wires: 


' 

Observer. 

Velocity in kilo- 
meters per second. 

Blond lot 

( 293,000 

1 298,000 
j 298,800 
} 300,300 
f 295,400 
299,400 

J 299,800 

1 299,800 
299,500 

L 299,900 

Trowbridge and Duane. .. . 

Saunders 



The average of the best determination of the velocity of light 
is about 299,900 kilometers per second, with which the alx>ve 
determinations of the velocity of the electric waves on copper 
wires is in good agreement. 

Velocity of Electric Waves in Air. — Although the velocity of 
the electric waves in air has not been determined by a direct 
method, the experiment of Sarasin and De la Rive showed that 
the velocity of the waves in air is the same as their velocity in 
copper wires surrounded by air, and therefore the same as that of 
light. 

Waves on Iron Wires. — On account of the magnetic properties 
of iron, the velocity of the waves on small iron wires has been 
found to be slightly less than the velocity of waves of the same 4 
period on a nonmagnetic metal like copper. With wires \ milli- 
meter in diameter and with 1 15, 000, 000 oscillations per second, 
St. John found that the velocity on the iron wire was 4 to 5% less 
than the velocity on the (‘upper wires. This result showed that 
the magnetization of the iron is able to follow extremely rapid 
reversals of the magnetizing current. 

On Surface Travel. — In addition to this slight change in veloc- 
ity due to the magnetic property of the iron, the damping effect 
of the resistance of the iron is very large. In attempting to esti- 
mate the effect of resistance on the damping of oscillations of high 
frequency, it should be remembered that these rapid currents 
travel in a very thin film on the outside of the conductor. By 




THE PROPAGATION OF ELECTRIC WAVES ON WIRES 71 


and A' B r and the spark gap i F. This circuit has its own definite 
period of oscillation. The other circuit, which we will call the 
“resonator circuit,” consists of the conductors gXX'g When 
the bridge X X ' is in the position that causes the Geissler tube to 
glow, the oscillator circuit and the resonator circuit are in reso- 
nance, and during one complete oscillation the electric wave goes 
from the bridge out to g', back across the bridge, out to g , and back 
again to the bridge* Whence it is seen that the length of the 
conductor from g ' across the bridge to g is the half wave length of 
the oscillator. 

If now the bridge is moved from XX 7 toward the oscillator, 
a second position SS' of the bridge is found for which the tube 
is caused to glow. During this displacement of the bridge, the 
self-inductsm.ee i and therefore the period, of the oscillator circuit 
is diminished, while the length of the wire to the right of the bridge 
is increased. Therefore, the wire to the right of the bridge cannot 
be in resonance, as a whole, with the oscillator circuit. We can 
show this experimentally, for if we leave the first bridge at SS' 
and place a second bridge across the wires, a position TT' can be 
found for which the presence of the second bridge does not affect 
the glow of the tube. A slight motion of the second bridge 11 to the 
right or to the left diminishes the glow. 

The two positions SS' and TT' are called nodes of electric 
potential. In a similar way with longer parallel wires several 
nodes may be located. The free end of the wires is always 
a loop of potential, and other loops of potential exist halfway 
between the nodes. The presence of these nodes and loops at 
equal intervals along the parallel wires shows the existence of a 
stationary wave system similar to that discovered by Hertz in his 
experiments with electric waves in air. 

Blondlot’s Apparatus. — A modification of Lecher’s apparatus 
made by Professor Blondlot is shown in Fig. 45. The two halves 
of the oscillator are here bent into semicircles, while the parallel 
wires lead out from a secondary circuit placed immediately be- 
neath the oscillator. The oscillator and the circular portion of the 
secondary are submerged in a glass vessel containing oil. Leads 
from the induction coil are brought into the oil and connected to 
the two sides of the spark gap, — one connection being made 
directly at a and the other connection being through a small 
spark gap at 6. In this form of apparatus the waves on the wires 
are produced by electromagnetic induction from the oscillator. 



THE PROPAGATION OF ELECTRIC WAVES ON WIRES 73 


In Paalzow and Rubens’s arrangement of apparatus (Fig. 48), 
in order to avoid disturbing the waves on the wires PQRS, the 
leads to the bolometer were not connected directly to the wires under 
examination, but were connected inductively by a single turn 
around capillary glass tubes TT, sliding on these wires. The 
glass tubes TT act as diminutive Leyden jars with the horizontal 
wires inside the tube for one coating, and the turn of wire on the 
outside of each tube for the other coating. Variations of electric 
potential at a point inside the little tubes induce (by electrostatic 
action) alternating potential in the turns of wire outside and 
produce alternating currents through one arm of the bolometer 
bridge. 

Figure 48 shows a form of apparatus suitable for experiments with 
this method. This is the form of apparatus used by Professor 



Fig. 48. Exploration of waves on wires by bolometer. 


St. John. As has been before mentioned, in order to get a simple 
stationary wave system in the parallel wires, these wires must 
have a proper length in comparison with the wave length of the 
waves. In St. John’s experiment the proper length of the wires 
was determined by trial. The exploring terminals of the bolom- 
eter were put at the ends P and S of the wires of Fig. 48. The 
oscillator was set in activity, and a reading of the bolometer was 
taken for this length of wire. A few centimeters of wire were cut 
off, and the reading again taken. This process was repeated until 
a maximum point was passed. A sharp and unmistakable maxi- 
mum was found when PQ had a certain length (859 centimeters). 
The effect fell off rapidly when the wires were shortened or length- 
ened from this point. The result is shown graphically in Fig. 49, 



CHAPTER XI 


WIRELESS TELEGRAPHY BEFORE HERTZ 

By Conduction, through. Water. — The first successful attempt at 
electric telegraphy 1 between stations not connected by wires seems 
to have been made by S. F. B. Morse in 1842. Morse describes his 
experiments in a letter to the Secretary of the Treasury of the United 
States, which was laid before the House of Representatives on Decem- 
ber 23, 1844. He says: 

“ In the Autumn of 1842, at the request of the American Institute, 
I undertook to give the public in New York a demonstration of the 
practicability of my telegraph, by connecting Governor’s Island with 
Castle Garden, a distance of a mile ; and for this purpose I laid my 
wires properly insulated beneath the water. I had scarcely begun 
to operate, and had received but two or three characters, when my 
intentions were frustrated by the accidental destruction of a part of 
my conductor by a vessel, which drew them up on her anchor, and 
cut them off. In the moments of mortification I immediately devised 
a plan for avoiding such an accident in the future, by so arranging 
my wires along the banks of the river as to cause the water itself to 
conduct the electricity across. The experiments, however, were 
deferred till I arrived in Washington; and on December 16, 1842, 
I tested my arrangement across the canal, and with success. The 
simple fact was then ascertained that electricity could be made to 
cross the river without other conductors than the water itself; but 
it was not until the last Autumn that I had the leisure to make a 
series of experiments to ascertain the law of its passage. The follow- 
ing diagram will serve to explain the experiment : 

“A, B, C, D (Fig. 51) are the banks of the river; N, P, is the 
battery; (7 is the galvanometer; ww, are the wires along the banks 
connected with copper plates,/, g, h, i, which are placed in the water. 
When this arrangement is complete, the electricity, generated by the 
battery, passes from the positive pole P, to the plate h, across the 

1 A large part of the historical information contained in this chapter was 
obtained from Mr. J. J. Fahie’s excellent History of Wireless Telegraphy, 
Dodd, Mead & Co., 1902. 


75 



WIRELESS TELEGRAPHY BEFORE HERTZ 




telephone in the circuit. In the first boat, which was moored, I 
kept a man making signals; and when my boat was near his I would 
hear those signals very well — a musical tone, something of this kind; 
turn, turn, turn. I then rowed my boat down the river, and at a 
distance of a mile and a quarter, which was the farthest distance I 
tried, I could still distinguish those signals.” 

In these experiments of Morse, Lindsay, Trowbridge and Bell the 
signals were carried from one station to the other by conduction 

through the water. The current in ^ 

flowing from one submerged plate to S’ 

the other at the sending station spreads / ^ \ 

out through the water in curves like / ^ \ 

those of Fig. 52. If, now, the termi- S / \ i 

nals of the receiving circuit dip down \ \ / \ j J y 

into the conducting area, the current 

divides, — part going through the water 

and part through the receiving circuit, / / | \ ^ / 1 \ 

in the inverse ratio of their resistances. j \ / \ 

This method of signaling, though at- 1 ^ S* 1 

tempted with improved apparatus ' 

by Messrs. Rathenau, Rubens, and FlG * 52 * LineH of ttow * 
Strecker, and by the latter carried to a distance of 14 kilometers (8.7 
miles), has not contributed to the art of wireless telegraphy, as it is 
now practiced*. 

Dolbear’s Apparatus. — A somewhat more suggestive apparatus was 
invented by the late Professor Dolbear of Tufts College, Massachu- 
setts, and was awarded a United States patent in March, 1882. Figure 
53, taken from the patent specifications, shows a diagram of the 
apparatus. The transmitting station, shown at the loft, consisted 
of a condenser H' connected to one terminal of the secondary of an 
induction coil G , of which the other terminal of the secondary was 
grounded at C. The primary of the induction coil contained a bat- 
tery / ' and microphone transmitter T. The receiving apparatus, 
shown at the right, consisted of a telephone receiver Ii with one 
terminal connected to ground at I), and the other terminal connected 
to a condenser H , which was in turn connected through a battery 1 B 
with a second condenser H 2 . 

Professor Dolbear, in his patent specifications, describes the action 
of the apparatus as follows : 

“ Now if words be spoken in proximity to transmitter T, the vibra- 
1 The function of this battery is not evident. 


nt - —-—W" 

;er — 

it, // V" — " /)\ ' 

r>s. \ \ ^ i \ 


Fig. 52. Lines of flow. 



WIRELESS TELEGRAPHY BEFORE HERTZ 


79 


Marconi had made the way clear, was made by Sir William 
Preece, engineer-in-chief of the postal telegraph system of Eng- 
land. Preece attempted to utilize the electromagnetic induction 
between two long horizontal wires, one at the sending station and 
the other at the receiving station. These horizontal wires were 
supported parallel to each other on telegraph poles, and were 
grounded at their two ends. The sending wire contained a battery 
and an interrupter, or else an alternating current generator, so that 
the line was traversed by an interrupted or an alternating current'; 
while the receiving circuit contained an ordinary telephone re- 
ceiver. The surging current in the sending wire produced a vari- 
able magnetic field surrounding it. This variable magnetic field 
produced by the sending circuit cut or linked with the receiving 
circuit, and induced a periodic electromotive force in it, which was 
evidenced by sounds in the receiver. 

After several years of experimenting, Sir William Preece was 
able to utilize this apparatus for signaling to some of the islands 
a short distance off the coast of England, and in 1898 a regular 
installation was established at Lavemock Point on the mainland 
and at Flatholm in the Bristol Channel, 3.3 miles (5.2 kilometers) 
apart. 

Preece’s experiments can be said to have availed only to show 
the futility of the attempt to get inductive action at long distance 
without the use of oscillations of high frequency , 



WIRELESS BY HERTZIAN WAVES — M ARGON!, 1896-1898 81 


many interesting experiments in the effort to obtain an explana- 
tion of its action. Branly’s radioconduetor is now familiarly 
known as the “ coherer,” — a name invented by Sir Oliver Lodge. 

Coherer, Applied to Study of Electric Waves. — In 1893 and 
1894 Sir Oliver Lodge applied the coherer to the study of electric 
waves by putting it in the place of the micrometer spark gap in 
a Hertz resonator, as is shown in Fig. 54. Under the action of the 
electric waves sent out from a properly placed Hertz oscillator, 
the resistance of the metallic filings in the coherer fell to a low 
value, so that the galvanometer (7\connected in series with a 
battery B in a lochl circuit through the coherer gave a deflection. 
After the waves ceased the resistance of the coherer remained low, 
so that the galvanometer remained deflected. Ia order to prepare 



Fia. 54. Sir Oliver Lodge’s apparatus for detecting electric waves. 

for another reading it was necessary to restore the filings to high 
resistance by tapping the tube. Lodge effected this restoration 
either by a tapping mechanism driven by clockwork, or by an 
electric trembler (like an electric bell) mounted on the same base 
as the coherer. 

With this apparatus Professor Lodge succeeded in detecting 
Hertz waves at a distance of about 55 yards from the source. 

Experiments of Popoff. — A still nearer approach to an operable 
form of receiving apparatus for wireless telegraphy was made in 
1895 by Professor Popoff of Kronstadt, and a description of the 
apparatus was communicated by him to the Physico-Chemical 
Society of St. Petersburg in April of that year. Popoff’s apparatus, 
which was designed for use in the study of atmospheric electricity, 
is ishown in the diagram of Fig. 55. The left-hand terminal of 
the coherer was connected, to a metallic rod extending above the 
house-top; the right-hand terminal of the coherer was connected to 
earth; so that electric currents produced by the atmospheric elec- 





WIRELESS BY HERTZIAN WAVES — MARCONI, 1896-1898 83 


Marconi’s 1896 Apparatus, — We come now to tlie early work of 
Marconi- After having made some preliminary experiments on his 
father’s estate near Bologna in Italy, Signor Marconi went to Eng- 
land, and on June 2, 1896, filed in the Patent Office of Great Britain 
a part of his first application for a patent for “improvements in trans- 
mitting electrical impulses and signals, and in apparatus, therefor.” 
The part of the application filed at this "date is without diagrams, 
and contains only provisional specifications . A complete speci- 
fication covering the same subject matter, amply illustrated with 
drawings and full of details as to the invention, was filed March 2, 
1897. This patent application of Mr. Marconi contains the first 
published account of a completed apparatus for successful wireless 
telegraphy by electric waves, and is, therefore, a document of con- 
siderable interest. It would seem to be not unprofitable to give 
careful attention to Marconi’s description of his invention. 

In the description that follows, the quotations are taken from the 
Marconi patent specifications; and after some of the paragraphs of 
quoted or paraphrased description I have added a brief paragraph 
in the form of a summary. 

Hertz or Righi Oscillator and Receiver. — At the transmitting 
station he employs “ a Ruhmkorff coil having in its primary circuit 
a Morse key for starting or interrupting the current.” The secondary 
of the coil he connects to “ pole appliances ” for producing the desired 
oscillations. Under “ pole appliances ” he mentions “ insulated balls 
separated by small air spaces or high vacuum spaces, or compressed 
air or gas, or insulating liquids kept in place by a suitable insulating 
material, or tubes separated by similar spaces and carrying sliding 
discs.” 

This form of the transmitting apparatus, as may also be seen by 
reference to the original drawings, is an ordinary Hertz or Righi 
Oscillator, actuated by a Ruhmkorff coil with a Morse key in its 
primary circuit. There is, however, also the suggestion of the 
use of a high vacuum or compressed air or gas about the spark 
gap. 

“ At the receiving instrument there is a local battery circuit con- 
taining an ordinary receiving telegraphic or signaling instrument 
and an appliance for closing the circuit.” The appliance for closing 
the circuit “ consists of a tube containing conductive powder, or 
grains, or conductors in imperfect contact, each end of the column 
of powder or the terminals of the imperfect contact or conductor 
being connected to a metallic plate of suitable length so as to cause 



WIRELESS BY HERTZIAN WAVES — MARCONI, 1896-1898 85 

have shown that the earthing of the circuits, though a convenience 
in construction, is not essential. 1 

It was with these earthed circuits that Marconi made his first 
great gains in the distance of transmission; but as we now look back 
over the experiments, we see that the gain in distance came about 
primarily through the fact that with this apparatus his circuits were 
placed vertical rather than horizontal, and also through the use of 
longer waves and more energy and larger radiating and receiving 
antennae, rather than through the use of the mere earth connections. 
To this subject we return in Chapter XIV. 

Marconi’s Coherer. — In addition to the practical introduction 
of the vertically placed radiator with ground connection Signor Mar- 
coni also made tremendous progress over other early investigators 
in his skill in constructing and using the coherer. A sketch of the 
coherer, drawn natural size from Marconi’s specifications, is shown in 
Fig. 58. The metal plugs PP 
are of silver slightly amalgamated 
with mercury, but no excess of 
mercury in the form of globules 
is left on them. The plugs fit 
accurately into a glass tube, and 
are within -pa inch of each other. The filings in the space between 
the plugs are preferably 96% nickel and 4% silver, and should not 
be fine, but rather coarse. They should be dry and free from grease 
and dirt, and should be uniform in size. The tube cont ainin g the 
filings is preferably exhausted of air and sealed up. In sealing up the 
tube care should be taken not to oxidize the filings. In order that 
the coherer may not be injured by the current through it, not more 
than t of an ampere of current should be used in the local circuit. 

Marconi’s Decohering Device. — One of the greatest difficulties to 
be overcome in operating a delicate coherer arises from the fact that 
the signal causes the coherer to become conductive, and if left alone, 
the coherer perseveres in this conductive condition. In order to 
restore it to its high resistance so as to be ready for the next signal, 
it is necessary to employ an automatic tapper, or trembler, which is 
started into action by the incoming impulse, and which stops the 
signal and itself when the incoming impulse ceases. Signor Marconi 
brought the decohering device to a high state of perfection, and as a 
result changed the capricious tube of filings into a reliable instrument 
for practical use. 



Fig. 58. Marconi coherer 
(natural size). 


1 Sec Chapter XIV. 



WIRELESS BY HERTZIAN WAVES — MAKCONI, 1896-1898 87 


spark. We have thus in our supposed case 100 sparks a second. 
Each spark occurs with oscillations of very high frequency and 
produces a train of electric waves. With such a sending station 
we should have arriving at the receiving station a train consisting 
of a few 1 of these extremely rapid waves, followed of a second 
later by a second similar train; and thus at intervals of second 
there would arrive successive short trains of waves while the send- 
ing key is depressed. 

Under the action of these trains of incoming waves the filings 
in the tube cohere, so that a current flows from the battery B i 
(Fig. 59) through the coherer Co and the relay R. This battery 
current pulls the armature of the relay so as to close the gap at 
A . When the gap is closed a second battery B sends a current 
through the coils of the sounder S, and also through the coils of 
the trembler T , The trembler is like an electric bell (with a 
somewhat shorter striking arm), and makes a series of strokes 
against the tube of the coherer. This decoheres the filings, but 
so long as the key at the sending station is closed, the waves 
continue to arrive and cause a repetition of the coherence, thus 
putting the coherer in a state of repeated coherence and decoher- 
ence during the arrival of the waves. The armature of the relay 
is adjusted so that the relay is somewhat sluggish and does not 
open at each decoherence. Therefore, the contact of the relay 
remains closed, and consequently the sounder armature stays 
down as long as the trains of waves continue to arrive. When, 
however, the sending key is released, and the waves cease to arrive, 
the decoherence due to the tapper perseveres, the relay contact 
opens, the sounder arm is released, and at the same time the 
trembler stops. Thus each closing and opening of the key at the 
sending station produces a corresponding down ami up stroke of 
the sounder, making a dash or a dot, according as the sending 
key is depressed for a long or a short interval of time. 

Instead of the sounder for translating the message an ordinary 
Morse registering tape-machine may be used to give a written 
record of the dashes and dots. 

Marconi's Protective Resistances and Inductances. — Return- 
ing now to diagram Fig. 59, let us examine into the purpose of the 
resistances p, p lf q , q ', and h , represented by the dotted lines. 

1 I have taken revolving-mirror photographs of the spark of a Marconi 
Oscillator with a period of xsffiffinF second, and found that there are about 
12 waves in a train. 



WIRELESS BY HERTZIAN WAVES— MARCONI, 1896-1898 89 


Balloons or Kites. — Another important suggestion contained 
in the 1897 specifications is the suggestion that “ the larger the 
plates of the receiver and the transmitter, and the higher from the 
earth the plates are suspended, the greater is the distance at which 
it is possible to communicate at parity with other conditions.” 
“ Balloons can also be used instead of plates on poles, provided 
they carry up a plate or are themselves made conductive by being 




Fkj. 60. Simple Marconi circuits with antenna sustained by a kite. 

Switch for “cutting over” from sending to receiving. 

covered with tinfoil. As the height to which they may be sent is 
great, the distance at which communication is possible becomes 
greatly multiplied. Kites may also be successfully employed if 
made conductive by means of tinfoil.” This sentence, therefore, 
provides for the use of antenna of great height. It should be 
noted here that the plates or tinfoil covering on the balloons or 
kites, which the inventor makes a necessary provision of the 
apparatus, are really nonessential. 

A diagram of circuits in which kites are used for suspending the 
vertical wires is shown in Fig. 60. 





WIRELESS BY HERTZIAN WAVES - MARCONI, 1896-1898 91 

Marconi’s Achievements between 1896 and 1898 . — In July, 1896, 
soon after arriving in England, Mr. Marconi submitted his plans to 
Sir William Preece, director of the postal-telegraph system of Eng- 
land. Preece, of whose activity in connection with attempts at 
wireless telegraphy we have already learned, entered eagerly into 
the new experiments. 

The first messages were sent from a room in the General Post 
Office to an impromptu station 100 yards distant. Soon afterwards, 
at Salisbury Plain, with parabolic reflectors about the instruments, 
communication was established at a distance of two miles. In May, 
1897, discarding the reflectors and using grounded circuits, Mr. 
Marconi covered a distance of 8.7 miles between Lavemock Point 
and Brean Down. Kites were employed in this experiment to sup- 
port the vertical wires. 

In July, 1897, important trials were made at Spezia, Italy, at the 
request of the Italian Government, and communication was estab- 
lished at a distance of 12 miles between a warship and a shore station. 

In July, 1898, the Marconi apparatus was used to report the yacht 
races at the Kingston Regatta, and a large number of correct mes- 
sages were exchanged between a press boat and the shore at dis- 
tances extending up to 20 miles. 

These various experiments constituted a complete demonstration 
of the utility of the invention. 



ELECTRIC WAVE TELEGRAPHY BY RESONANT CIRCUITS 93 


A Simple Variable Circuit. — A simple method -of easily varying 
the period of a receiving circuit consists in the use of a variable in- 
ductance L (Fig. 62), inserted between the detec- 
tor and the antenna or between the detector and 
the ground at the receiving station. Such a vari- 
able inductance, or tuning coil, is made of a single 
layer of wire wound on an insulating tube of glass 
or ebonite, and is varied by a contact sliding along 
the coil so as to put more or fewer turns of induct- 
ance into the circuit. A similar tuning coil, 
though usually of larger wire, may be used at the 
sending station also. At the sending station the 
coil is inserted between the spark gap and the an- 
tenna or between the spark gap and the ground 
connection. 

Increase of inductance in either circuit increases 
the time of vibrations, which brings a correspond- 
ing increase of wave length. 

The use of adjustable inductances in both the 
sending and the receiving circuits was apparently Fl antenna, S 1 circuit 
first suggested by Sir Oliver Lodge in a patent having a variable 

application of 1887, which is reviewed later in the tuning^ 06 f ° r 

present chapter. 

Coupled Circuits. — ( Vrtain other methods, employed for adjust- 
ing both the sending station and the receiving station, and found to 
produce better results both for transmitting with large quantities of 
energy and for receiving with comparatively sharp resonance, make 
use of coupled circuits . The resonance relations in these coupled cir- 
cuits has been the subject of much theoretical and experimental 
research. As introductory to the description of the coupled 'circuits, 
I shall recall to the reader the familiar and interesting experiments of 
Mr. Tesla and of Professor Thomson on the production of electric 
oscillations of high frequency and high potential. 

High-frequency Transformers of Thomson and Tesla. — The 
high-frequency transformer that was apparently independently 
developed by Mr. Nikola Tesla and Professor Elihu Thomson about 
1890 is shown in sketch in Fig. 63. A primary coil P, consisting of 
one or two turns of heavy wire, is connected in series with a bank 
of Leyden jars C and a spark gap G. A secondary coil S , consisting of 
three or four hundred turns of wire wound in a single layer on a paper 
or vulcanite tube, is inserted axially within the primary. When the 





ELECTRIC WAVE TELEGRAPHY BY RESONANT CIRCUITS 95 


circuit and the secondary coil snail ue m resonance with each 
other. 

By the use of apparatus of this character Mr, Tesla has pro- 
duced enormous sparks — twenty-three feet long and of great 
volume — graphically described as being accompanied by a roar 
like Niagara. 

The transformer PS is called a high-frequency transformer , an 
oscillation transformer , or an air-core transformer , to distinguish it 
from an ordinary iron-core transformer, such as is used with 
commercial alternating currents of slow frequency. 

Oscillation transformers, built on somewhat different lines from 
the one above described, have met with application to both the send- 
ing and the receiving circuits of electric- wave telegraphy, and by the 
use of these transformers a considerable advance has been made, 
both in the greater distances attained and in the diminished con- 
fusion of signals of different wave lengths. 

Two Systems of Coupled Circuits. — The form given to these 
coupled circuits is considerably varied in practice. There are, 
however, two important general types. These are represented in 
the accompanying figures (Fig. 64 and 65) and are called re- 
spectively the inductively coupled and the direct coupled types. 

The Inductively Coupled Type. — This type is shown in Fig. 64, 
In this system the sending station, shown on the left, is seen to 
consist of a Tesla high-frequency apparatus, with one secondary 
terminal connected to an antenna and the other secondary ter- 
minal connected to the ground. Power is supplied to the circuit by 
an alternating current transformer or a Ituhmkorff coil to which 
the wires W , W' lead. 

The receiving station of this system, shown at the right in 
Fig. 64, has also an oscillation transformer P' S', and is in prin- 
ciple like the sending station, except that the detector D' with its 
accessories is usually put in place of the spark gap of the sending 
apparatus. The coils P' and S' and condenser C' used with the 
receiving apparatus generally have different inductances and 
capacity from those of the sending apparatus, and not being 
traversed by high-potential currents they are usually made more 
compact. 

In this inductively coupled system of circuits, oscillatory cur- 
rents in the sending antenna are produced inductively by the 
oscillatory discharge of the condenser C through the primary coil 
P. These oscillatory currents in the sending antenna produce 



ELECTRIC WAVE TELEGRAPHY BY RESONANT CIRCUITS 97 

system employs auto-transformers; that is to say, instead of hav- 
ing separate primary and secondary coils in the high-frequency 
transformer, the primary coil (P or P') at either station is a part 
of the secondary coil. At the sending station (at the left) the 
condenser discharges through some of the turns of the secondary, 
and the discharge acts inductively on the -whole of the secondary. 
Likewise, at the receiving station the oscillations in the antenna 
pass through a part P r of the secondary S' and act inductively on 
the whole of S'. Theory and experiment show that in principle 
the direct coupled circuits differ very little from the inductively 
coupled system. 

Introduction of Coupled Circuits into Practice. — Postponing 
for a time the direct discussion of the principles involved in the 
use of the coupled circuits, let us take up historically the matter 
of the introduction of these circuits into wireless telegraph practice. 
The examination of the question as to the priority of the different 
claimants to this improvement is fraught with considerable diffi- 
culty. Lodge, and Marconi in England, and Braun in Germany, 
have clearly established dates of publication by patent applica- 
tions. While examining the question of priority, I shall also give 
a brief description of the apparatus of these several inventors, so 
far as pertains to the form of circuits used. 

Sir Oliver Lodge’s Apparatus. — On May 10, 1897, Professor 
Ijodgo filed a patent application in England for improvements in 



wireless telegraphy. The corresponding application in the United 
States was filed Feb. 1, 1898. What he claims to bethe most promi- 



ELECTRIC WAVE TELEGRAPHY BY RESONANT CIRCUITS 99 


charging into the antenna circuit, and if the coil k has a large 
inductance, as it seems to have from the fact that it is made of 
ct fairly thin wire,” this sending arrangement may be looked upon 
as a special and very imperfect form of the direct-coupled type of 
sending circuit of Fig. 65. It is imperfect in the use of the mul- 
tiplicity of spark gaps, for if all the gaps except h l0 h 11 had been 
closed, the coil k, which was put in as a charging bridge across the 
unnecessary gaps, could then have been omitted, and the apparatus 
would have been a very useful form of direct-coupled emitter. 

While we are accustomed to the use of multiple gaps in replace- 
ment of a single gap, and while the multiple gap is in some construc- 
tions a distinct advantage over the single gap, still the introduction 
of one of the multiplicity of gaps directly into the antenna cirfi«i»e 



Fro. 08. Lodge’s inductively coupled receiving transformer. 

is certainly an annulment of the chief advantage accruing from the 
coupled circuits. 

In the receiving apparatus Lodge shows the use of an oscillation 
transformer. Reference is mack 1 to Fig. 68. His conical capacity 
areas or their equivalent are connected to the primary coil h 4 . About 
this a secondary coil u is placed, and is connected with the coherer e, 
a battery/, and the telegraphic receiving instrument g. The purpose 
of connecting the detector in a secondary circuit instead of directly 
in the antenna, is, according to the patentee, to “ leave the resonator 
freer to vibrate electrically without disturbance from attached wires.” 
This is an excellent reason, but the receiving apparatus, as shown in 
this diagram, which was taken from Lodge's patent specifications, 
has the fatal defect that no condenser is shown in the secondary 
circuit, and that the high-frequency oscillations have to go through 
the telegraph instrument. Hence, apart from the suggestion of the 



ELECTRIC WAVE TELEGRAPHY BY RESOJSTANT CIRCUITS 101 


inductance L added below the gap, 
for preserving approximate electrical 
symmetry and for tuning. 

In addition to these various sugges- 
tions by Lodge in regard to the use of 
timing coils and transformers in the 
circuits, and the maintenance by him 
of the possibilities of the ungrounded 
circuits, Professor Lodge, together with 
Messrs. Muirhead and Robinson, has 
also devised a new form of coherer. 
This is described in Chapter XVI. 

The Coupled Circuits of Ferdinand 
Braun. — Let us return to the matter 
of the coupled circuits. In a German 
patent, No. 111,578, applied for October 
14, 1898, Professor Ferdinand Braun of 
Strassburg in Germany descrilies “ a 



Fig. 70. Elevated conductor 
and ground of wire netting 
(Lodge). 




Fig. 71a, 716, 71c, 71 d. Professor Ferdinand Braun’s methods of coupling a 
condenser circuit to an antenna. 



ELECTRIC WAVE TELEGRAPHY BY RESONANT CIRCUITS 103 


patent, filed Feb. 6, 1899, does Professor Braun speak of the neces- 
sity of properly attuning the secondary circuit to the period of the 
condenser circuit, which is a prerequisite for attaining the high poten- 
tial in the antenna circuit, and without this attuning of the secondary 
to the primary circuit, the large capacity of the primary condenser 
and the use of powerful sources of electricity would not give any 
advantage over the simple Marconi antenna. 

The first mention by Braun of the required tuning, so far as 
I have been able to find, is in a publication of the 5th of March, 
1901, in the Physikalische Zeitschrift, Vol. 2, p. 373, and in a book 
by Braun entitled Drahtlose Telegraphie durch Wasser und Luft, 
published in 1901. 

In examining Braun’s patent drawings one may wish to know 
whether the antenna circuit is to be grounded or otherwise bal- 
anced by a capacity at the other end of the secondary. Nothing 
is said on this subject in the German patent, but in the correspond- 
ing American patent he says, with reference to Fig. 71 rf, that 
“ one end of the secondary coil of the transformer & is connected 
with the transmitting wire M, and the other 
end is shown prolonged and ending in an 
arrow to indicate that it may be prolonged 
by adding a suitable length of insulated wire or 
connected to some other capacity area.” In 
his book of 1901 a corresponding prolongation 
or addition of capacity is indicated in his draw- 
ing of the direct coupled circuit, as is shown in 
Fig. 72. 

There is nothing in these early patents of 
Professor Braun relating to coupled circuits at 
the receiving station. The coupled receiving 
circuits were undoubtedly invented by Mar- 
coni, and also his description of the induc- 
tively coupled sending station, though of pub- 
lished date a little later than that of Braun, is 
a much fuller and a more complete disclosure 
of the invention. The work of Marconi in developing the 
coupled circuits will now be discussed. 

Marconi’s Coupled Circuits. — Mr. Marconi, in an English 
patent applied for June 1, 1898, clearly sets forth the transformer 
arrangement for a receiving station . This is shown in Fig. 73, in 
which A leads to the antenna, and E to earth. The coils JV 2 , 



Ficj. 72. Form of 
direct coupled 
transmitter de- 
vised by Fer- 
dinand Braun. 



ELECTRIC WAVE TELEGRAPHY BY RESONANT CIRCUITS 105 

of each other — that is to say, the electrical time periods of the 
four circuits are to be the same or octaves of each other.” 

The advantages of the coupled circuit at the sending station, 
according to Signor Marconi, arises in “ the approximately closed 
circuit of the primary being a good conserver and the open cir- 
cuit of the secondary being a good radiator of wave energy.” 

The variable inductance g in Fig. 74 placed in the antenna of 
the sending circuit and a corresponding coil at the receiving 
station were used to aid in this process of tuning. 



Fig. 75. Apparatus used by Marconi for sending two messages at onee. 

A sending and a receiving station devised by Mr. Marconi for 
sending or receiving two messages at once with the use of a single 
antenna are shown in Fig. 75 and Fig. 76. This was successfully 
employed and exhibited by Marconi in the autumn of 1900. 
Two operators at the two keys K and K of the sending station 
made the signals. The two condenser circuits having different 
values of capacity and self-inductance were independently charged, 
and discharged with different periods of oscillation- These two 
periods were both impressed on the antenna through circuits 
which by means of the antenna inductances d and d! were made to 
resonate with the respective condenser periods. At the receiving 



ELECTRIC WAVE TELEGRAPHY BY RESONANT CIRCUITS 107 


supported by a kite. The detector employed in the transatlantic 
experiments was an instrument known as the “ Italian Navy 
Coherer,” and consisted of a globule of mercury between iron 
terminals in a glass tube. This form of detector is self-restoring, 
and with it a telephone receiver in series with a battery is used in 
the local circuit in the place of the ordinary telegraph relay. 

In March, 1902, messages sent out from Poldhu were received 
by the Marconi apparatus on board the steamer Philadelphia 
when the steamer was 1550 miles (2400 kilometers) from the send- 
ing station. In December of the same year Marconi announced 
the transmission of three entire messages from Glace Bay, Nova 
Scotia, to Poldhu in England, a distance of 2300 miles. 

On January 19, 1903, the powerful Marconi station at Well- 
fleet, Cape Cod, Massachusetts, transmitted to Poldhu, England, 
the following message from the President of the United States to 
the King of England: 

“His Majesty, Edward VII, 

London, England. 

In taking advantage of the wonderful triumph of scientific research and 
ingenuity which has been achieved in perfecting a system of wireless teleg- 
raphy, I extend on behalf of the American people most cordial greetings and 
good wishes to you and to all the people of the British Empire. 

THEODORE ROOSEVELT.” 

W klli'i.ket, Mass., 

January 10, 1903. 

This message, though intended to be relayed at Cape Pace, was 
received, according to reports issued by the Marconi Company, 
direct at the Poldhu station in England. 



NATURE OF THE OSCILLATION 


109 


If the electrostatic capacity per unit of length of the rods is uni- 
form throughout both rods, which is approximately true, when the 
rods are not too short the potential of the conductor at any point of 
its length will be proportional to the charge, so that the shaded area 
representing a distribution . of positive charge may also be looked 
upon as showing the distribution of positive 'potential, while the 
unshaded area represents negative potential. Thus, in the condition 
depicted in diagram (a), there is a uniform positive potential over 




Charged 
top 4 - 



oo 

Current 
-- () 


c*o 

Current 
Max. down 


c£> 


ent 

O 


CO 

Current 
Max. up 


CO 

Current 
— O 


Fig. 77. Potential and current distribution. 


the top rod, and a corresponding negative potential over the bottom 
rod. This is before the* spark begins. 

Supposes now, the spark to start between the rods; the gap between 
the rods becomes conductive, and a current begins to flow between 
the rods. There is a flow of positive electricity from the top rod 
and a flow of negative electricity from the bottom rod. The elec- 
tricity to flow first across the gap is that in the neighborhood of the 
spark gap, because it is there that the potential gradient is greatest. 
After a short time — one-fourth the period of a complete oscillation 
— the condition of the charge, and likewise the potential, of the rod 



NATURE OF THE OSCILLATION 


111 


because any diminution of the current diminishes the surrounding 
magnetic field, and gives an electromotive force tending to preserve 
the current. The current thus continues to pile up a positive charge 
on the lower rod, in spite of the fact that this piled-up charge is 
exerting a restoring force. 

Presently, however, this restoring force, which has gone on increas- 
ing, brings the current to a stop. Then when there is no current, 
there is no magnetic field, and the accumulated positive electricity 
on the lower rod starts the current upward. This reversal of the 
current occurs at a time t = Tf 2; and the condition of the charge 
and current is represented at (c) and (o'). The upward current 
continues to flow, and produces successively the conditions (d) and 
(cO) at t = 37Y4, and (e) and (e/) at t — T. 

In the last named state the upper rod is entirely positive, while 
the lower rod is entirely negative. This resembles the initial 
state of the rod, but is not identical with it, because the initial 
state was brought about by an extraneous slow charging source 
(Holtz machine or Ruhinkorff coil) instead of by the very rapid 
surging that is going on in the oscillator when it is oscillating with 
its own natural period. 

From the condition of initial uniform distribution we have 
followed the charge and current, by rather large stages of a quarter 
of a period each, through a single oscillation. The charge on the 
conductor will continue to oscillate, going through the successive 
stops several times — the accumulation of electricity becoming 
less and less at each oscillation until the spark extinguishes. 

Nodes and Loops of Potential and Current. — From the pre- 
ceding discussion it is apparent that the two ends of the Hertz 
oscillator undergo maximum fluctuations of potential, and are, 
therefore, loops of potential . The middle of the conductor during 
the oscillation has no accumulation of charge on it; the potential 
of the middle, therefore, never rises above zero (after the start), 
and is a node of potential. 

On the contrary, the nodes of current arc 4 at the ends of the oscil- 
lator, while a loop of current is at the middle of the oscillator. 

EXPERIMENTS ON THE DISTRIBUTION OF CURRENT IN AN 
UNGROUNDED HERTZ OSCILLATOR 

In the preceding dicussion there was given a theoretical exami- 
nation of the nature of the potential and the current distribution 
occurring in a Hertz oscillator. I have recently made a simple 



NATURE OF THE OSCILLATION 


113 


L , 30 cm. on a side, and having in series with it a variable con- 
denser C and a high-frequency current-reading instrument at J. 
I shall now describe the instrument I and the condenser C. 

Description of the Instrument. — The instr um ent at I as is 
shown in Fig. 80 consists of a disc of silver, suspended by a fine 
fiber of spun quartz so as to hang near a small coil of a few 
turns of wire, with which the disc made an angle of 45 degrees. 
The disc is at M, and the coil, which in this experiment consisted 
of five turns of wire wound on a vulcanite tube, is shown at C, 
Fig. 80. The two ends of the coil are connected to binding posts, 
n by which the coil is put into the circuit. 

The front of the disc carries a small 
mirror, enabling the deflections of the 
disc to be measured by means of a 
telescope and scale such as is used with 
delicate galvanometers. 

The mounting of the instrument is also 

shown in Fig. 80. 
The disc is sus- 
pended in the 
vertical vulcan- 
ite tube, which 
is mounted on 
leveling screws ; 
the support of 
the coil is in- 

Fig. 80. High-frequency dynamometer. Mounting shown sorted in the side 
at left, suspension at right. Q f the vertical 

tube, and is arranged to be moved in and out by a micrometer 
screw. This delicate, motion of the coil in or out brings the 
coil nearer to or farther from the suspended silver disc so as to 
vary the sensitiveness of the instrument, to make it suitable for 
measuring small or large oscillating currents. 

The action of the instrument, which we shall call a “ high- 
frequency dynamometer, 77 is as follows: oscillations in the coil 
induce oscillations in the disc. Between these two sets of oscilla- 
tions there is a force which causes the disc to tend to set itself 
at right angles to the coil. 1 The deflections of the dynamometer 
are proportional to the square of the current through it. 2 

The principle of this instrument was independently discovered by Dr. 
Elihu Thomson and by Professor Fleming. The instrument was first shown 





NATURE OF THE OSCILLATION 


115 


this character, with capacity somewhat larger than that of the 
condenser employed in these experiments, is shown in Fig. 81. 

Large Current at Resonance. — Variations of the capacity of 
C varies the natural period of oscillation of the condenser circuit, 
and when this period is made equal to that of the Hertz oscillator 
OOO, a maximum deflection of the instrument 1 is obtained, under 
the action of the oscillation. 

The resonant condenser circuit when calibrated in terms of 
wave length is a form of “ wave meter.” How this calibration 
is effected will be shown later. 

Exploration of Current Distribution. — Since the wave meter 
in this form, on account of the instrument I, is not conveniently 
movable, it was necessary to move the oscil- 
lator in order to explore the distribution of 
current in the oscillator. The oscillator, with 
its exciting induction coil and storage bat- 
tery, was moved lengthwise, keeping it al- 
ways the same distance from the wave meter, 
by means of the vulcanite guides DD of 
Fig. 79. Readings of the dynamometer 
were taken for various positions of the 
oscillator with respect to the wave meter. 

This was equivalent to moving the wave 
meter along the oscillator, and the readings 
of the dynamometer were proportional to 
the square of the current in the wave meter, 
and therefore proportional to the square of 
the current at different points of the oscil- 
lator; because, the induced current, keeping 
everything else the same, is proportional to 
the inducing current. 

The results obtained for the distribution 
of the current in the oscillator are plotted 
in Fig. 82. The curve of Fig. 82 shows that 
the current in the oscillator is greatest near 
the gap and falls off to zero at the ends of 
the oscillator in a manner not very different 
from that shown in the theoretical drawings of Fig. 77. There 
is a loop of current in the middle and a node at each end of 
the oscillator. 



I I I I L_ 

• 3 .4 .6 .8 in 


Relative Current 

Fig. 82 . Distribution 
of current along a 
Hertz oscillator, as 
determined by ex- 
periment. 




NATURE OF THE OSCILLATION 117 


and we must therefore conclude that the ratio of X/Z for very short 
oscillators is greater than for the long oscillator. 

We are primarily interested in the long oscillators, and in order to 
extend the experimental records to the case of longer oscillators 
than those studied by Conrad, I have made a c 

series of measurements with the apparatus of 
Figs. 78, 79, 80. 

Calibration of the Wave Meter. — The wave 
meter was calibrated for various adjustments 
of the condenser C by setting it to resonance 
with various lengths of the two parallel wires 
of Fig. 83, as had been previously done by 
Drude. With the wave meter calibrated to read 
directly in wave lengths, the parallel calibrating 
wires were removed, and the Hertz oscillator, o B 

consisting of two oppositely extending wires Fig. 83. Parallel- 
of various lengths (1 mm. in diameter), was Smtingwlve- 
brought up near the wave meter, and the wave meter for short 
length produced by the oscillator was deter- wave-lengths, 
mined. The results obtained are given in the following table: 


AUTHOR’S TABLE OF RESULTS FOR RELATION OF X TO l 


l 

Half length of 
oscillator in 
meters. 

X 

Wave length in 
meters. 

\ 

l 

4.0 

16-9 

4.22 

4.5 

18.9 

4.20 

5. 

21.2 

4.23 

5.5 

23.2 

4.22 

6. 

24.9 

4.15 

7. 

29.5 

4.21 

8. 

33.6 

4.20 

9. 

38.7 

4.23 

10. 

41.6 

4.16 

11. 

46.1 

4.22 

12. 

49.5 

4.13 

13. 

53.9 

4.14 

14. 

57.5 

4.11 

15. 

63.0 

4.19 


Average 

4.19 


The average of the results obtained by the author for the ratio 
of X to Z, namely, X/Z = 4.19, for wave lengths between 17 and 
63 meters, is a little less than the corresponding ratio, 4.24, ob- 






NATURE OF THE OSCILLATION 


119 


If the capacity attached were very large (e.g. the earth), the 
point of zero fluctuation of potential would again be brought near 
the instrument, because a large fluctuation of potential c ann ot 
occur in a very large capacity under the action of the currents 
with which we are concerned. We should, therefore, have the 
same current as when the conductor was made up of two parts 
symmetrical about the instrument. 

In actual systems, the grounding may be imperfect. In that 
case the symmetrical image would give only approximately an 
equivalent system. 

I have made some experiments to test this image theory of the 
action of the ground connection. The experiments consisted in 
comparing resonance curves taken with various forms of grounded 
circuits with the corresponding resonance curves taken with an 
image circuit in the place of the ground. Two of these experi- 
ments are here briefly described. 

EXPERIMENTS TO TEST IMAGE THEORY OF THE GROUND 

Experiment I. The Aerial Circuit and its Imag e Timed by 
Variable Inductances. — In testing the image theory of the action 
of the ground at the receiving sta- 
tion the form of circuit shown in 


ft 

O 

ZJ 

Cl> 

OJ 


Fig. 84. Circuit employed in Fig. 85. Resonance curves in study of the 
study of the image theory of image theory of the ground. Curve H 
the ground. was obtained with horizontal duplicate 

of antenna; curve G, with ground. 

Pig. 84 was employed. The high-frequency dynamometer de- 
scribed on p. 113 was used for detecting and measuring the minute 
oscillating currents at the receiving station, and was placed at I 




1 2 3 4 5 6 7 
Inductance xlO _5 Henry 




NATURE OF THE OSCILLATION 


121 


The curve F, with which we are not here concerned, was obtained 
with the duplicate antenna wound around the house of the receiv- 
ing station. 

Experiment n. Quarter-Wave Ground. — What was perhaps 
a more interesting experiment confirmatory of the image theory of 
the ground was made by replacing the ground by a horizontal wire 
of which the length could be varied. The relative amounts of 
energy received (deflections) for different lengths of the horizontal 
wire are shown in the curve A of Fig. 86. Resonance was obtained 
when this wire had the length of 38 
meters, which was very close to one- 
fourth the wave length (153 meters). 

The ground gives the system the 
same period as an added quarter- 
wave wire gives the system. Curve 
B obtained with different conditions 
leads to the same results. 

Conclusion from the Experiments. 

— These experiments I and II show 
that the effect of the ground, so far 
as concerns the vibration in the an- 
tenna, is to introduce into the circuit 
at the ground a point of zero fluctu- 
ation of potential, — an effect that 
can also be obtained with an arti- 
ficial ground consisting of a sym- 
metrical duplicate of the aerial 
system or consisting of a horizontal 
wire not far from the earth and of 
length equal to one-quarter of the wave length to be received. 



Fro. 86. Showing that the 
ground may he replaced by a 
quarter-wave wire. 


Professor Ferdinand Braun at a date earlier than that of my 
experiments has suggested the use of horizontal wire in replace- 
ment of the ground and also the use of a capacity consisting of a 
large cylindrical conductor in the place of the ground. He has 
not, however, so far as I know published any quantitative results 
on the subject. 



NATURE OF THE OSCILLATION 


121 


The curve F, with which we are not here concerned, was obtained 
with the duplicate antenna wound around the house of the receiv- 
ing station. 

Experiment 13. Quarter-Wave Ground. — What was perhaps 
a more interesting experiment confirmatory of the image theory of 
the ground was made by replacing the ground by a horizontal wire 
of which the length could be varied. The relative amounts of 
energy received (deflections) for different lengths of the horizontal 
wire are shown in the curve A of Fig. 86. Resonance was obtained 
when this wire had the length of 38 
meters, which was very close to one- 
fourth the wave length (153 meters). 

The ground gives the system the 
same period as an added quarter- 
wave wire gives the system. Curve 
B obtained with different conditions 
leads to the same results. 

Conclusion from the Experiments. 

— These experiments I and II show 
that the effect of the ground, so far 
as concerns the vibration in the an- 
tenna, is to introduce into the circuit 
at the ground a point of zero fluctu- 
ation of potential, — an effect that 
can also be obtained with an arti- 
ficial ground consisting of a sym- 
metrical duplicate of the aerial 
system or consisting of a horizontal 
wire not far from the earth and of 
length equal to one-quarter of the wave length to 

Professor Ferdinand Braun at a date earlier than that of my 



Fro. 86. Showing that the 
ground may be replaced by a 
quarter-wave wire. 

be received. 


experiments has suggested the use of horizontal wire in replace- 
ment of the ground and also the use of a capacity consisting of a 
large cylindrical conductor in the place of the ground. He has 
not, however, so far as I know published any quantitative results 
on the subject. 




PROPAGATION OVER THE EARTH 


123 


pendicular to the direction of motion when the charge is brought 
up along the plane, and the work done is therefore zero. For 
further details in regard to work and potential see Appendix I. 

Having shown that the plane P is everywhere at zero poten tial, 
let us next introduce the idea well established in treatises on elec- 
tricity, that so long as we keep the potential of the plane P equal 
to zero the electric force in the region between A and the plane P 
is completely fixed, no matter what changes we may introduce 
below the plane. If, then, the lower half of the diagram is removed 
and the plane is in some other way kept at zero potential, the 
electric force between A and the plane will be the same as before; 
namely, that represented in Fig. 88, which is the upper half of 



Fig. 88. Lines of electric force between a 
charged body A and an infinite conducting 
plane kept at zero potential. 


Fig. 87. Wc may keep the plane at zero potential by grounding 
it so that it comes into coincidence with the surface of the earth; 
or the surface of the earth itself may take the place of the plane, 
provided the earth for a considerable area around the charged body 
A is a good conductor. 

That is to say, if the earth's surface is a good conducting plane 
for a considerable extent, and a charged body A be placed above 
the surface of the earth, the field of electric force between A and 
the plane surface of the earth will be the same as the upper half 
of the field between A and a body B, which has a charge equal to 
A and opposite in sign, — 7i being at the distance below the plane 
that A is above it. This equal opposite charge symmetrically 
placed in regard to the plane is called the electrical image of A in 
the plane. 

Similar Theory Applied to the Oscillator. — If we next consider 
the case of the electric oscillator, the field of electric force for the 
symmetrical oscillator, as we have seen in Chapter VIII, is roughly 
that represented in Fig. 89. The ideal, nonmaterial plane PP 
through the figure is at zero potential, so that the lower half of 
the diagram could be replaced by the surface of the earth, if it 
were plane and perfectly conductive, without disturbing the upper 



PROPAGATION OVER THE EARTH 


125 


are not visible from each other. We have here a simple view of 
the matter, obtained on the assumption that the earth is a perfect 
conductor. 

The Earth not a Perfect Conductor. — The surface of the earth 
is, however, not everywhere a good conductor of electricity. The 
sea and moist soil are better conductors than dry stone. In some 
places the surface materials of the earth are in fact good insulators. 

The attenuation of the electric wave is on this account very 
different over different parts of the surface of the earth, — condi- 
tioned on the fact that there is a greater or less penetration into 
the insulating portions and a greater or less absorption of energy 
at the poorly conducting portions. This subject has been sub- 
mitted to a very remarkable mathematical treatment by Dr. Zen- 
neck. The mathematical reader is referred to Dr. Zenneck’s 
paper 1 or to Professor Fleming's 2 translation and “ free para- 
phrase ” of it, for a beautiful discussion of this interesting question. 
I shall attempt to give here a brief statement of some of Dr. 
Zenneck's results without attempting to present his reasoning. In 
doing this I wish to acknowledge the assistance afforded by Pro- 
fessor Fleming's excellent commentary on Zenneck's paper. 

In order to simplify the matter, Dr. Zemieck at first considers 
only the case of a plane electric wave traveling without divergence 
over a flat surface. lie is thus at first leaving out of account the 
spreading out of the wave and the consequent diminution of ampli- 
tude by mere distance; and he is also omitting the attenuation of 
the wave due to the curvature of the surface. 

Instead of considering the earth to be a perfect conductor, as 
has usually been done before, Zenneck looks upon the boundary 
between the earth and the air as the boundary between two media 
of different conductivities and different dielectric constants; and 
he transforms Maxwell’s equations so as to take account of the 
two media. 

He arrives at the conclusion that where the earth is a good con- 
ductor (for example, xea water), the electric force (at the surface) 
is perpendicular to the surface. For waves of wave length 600 
meters, which is the wave length used in most of the calculations, 
sea water acts as a good conductor, and the electric force at the 
surface of the sea is perpendicular to the surface, as is shown in 

1 J. Zenneck: Annalen der Physik, Vol. 23, 1907. 

2 Fleming: Engineering (London), June 4 and 11, 1909. 



PROPAGATION OVER THE EARTH 


127 


considering a radius drawn from the center of the ellipse to a 
particle moving around the ellipse with the frequency of the wave. 
The length and the direction of the radius so drawn would repre- 
sent the changing magnitude and direction of the electric force. 
Such an electric wave, oscillating both in magnitude and direction 
is equivalent to two waves, one tending to produce vertical cur- 
rents and the other tending to produce horizontal currents (the 
two effects being also out of phase with each other). The hori- 
zontal oscillating force induces currents in the earth’s surface, and 
diminishes the energy of the progressing wave, so that in this case 
the distance to which signals can be sent is less than in the case of 
the good conductor. 

In the case of propagation over very dry soil, which is not so good 
an insulator as the rock (r = 10,000 ohms per meter cube, k = 1 
to 3) Zenneck finds the result represented in diagram (c), Fig. 91. 



Fig. 93. Curves taken from Professor Fleming’s commentary on Zenneck’s 

theory, from the Electrician. 

Although the conductivity in this case is between that of (a) and 
(b), the form of the ellipses is not intermediate between (a) and (b). 
The relation is not a simple one, involving resistance alone ; because, 
in fact, a perfect conductor and a perfect insulator give in the region 
above the surface the same form of unabsorbed, vertical wave; and 
there is an intermediate case of conductivity and dielectric con- 



PROPAGATION OVER THE EARTH 


129 


space, and would not be constrained at all to follow the curvature 
of the surface. 

From this it is clear that for the easy transmission of the electric 
waves between stations sufficiently separated to have a large por- 
tion of the earth’s curved surface between, what is required is a 
good conducting and not an insulating expanse for the waves to 
travel over. In the succeeding sections we shall compare the dis- 
tance of transmission over poor conductors with that over a good 
conducting expanse. To do this we must take into account the 
divergence of the waves with distance to see whether or not the 
absorption is important in any particular case. 

D imin ution of Amplitude by Divergence with Distance. — On 
account of the divergence of the waves from the sending station, 
the amplitude of the electric force in the wave is approximately 
inversely proportional to the distance from the oscillator, provided 
there is no absorption and provided the distance is not too small. 
This has been shown theoretically to be true in the case of the 
propagation of the waves in free space. This law has also been 
approximately verified for wireless telegraph weaves traveling over 
sea water for distances up to 60 miles, in a very beautiful set 
of experiments performed on the Irish Channel by Messrs. W. 
Duddell and J. E. Taylor. 1 

Messrs. Duddell and Taylor’s experiments consisted in receiving 
and measuring the current set up in the antenna of a shore station 
by electric waves sent out from the British telegraph repair ship 
Monarch , while the ship was at various distances from the receiving 
station. The ver> r minute currents received were measured by 
Duddell’s thermogalvanometer, of which the following is a brief 
description : 

The thermogalvanometer invented by Mr. W. Duddell 2 is in 
principle the Radio micrometer of Professor C. V. Boys, with a 
modification required to adapt it to measuring oscillatory electric 
currents instead of heat radiation, for which Boys’ instrument was 
designed. A diagram of the essential parts of the instrument is 
shown in Fig. 94. Between the poles NS of a strong permanent 
magnet is hung a small loop of one turn of wire L , by means of a 
very fine quartz fiber F . The loop is closed below by a thermal 
junction of bismuth Bi and antimony Sb . Heat applied in any 

1 Duddell and Taylor: Journal of the Institution of Electrical Engineers, 
Vol. 35, pp. 321-352, 1905. 

2 W. Duddell: Phil. Mag., Vol. 8, p. 91, 1904. 



PROPAGATION OVER THE EARTH 


131 


Monarch. In these curves the product of received current times 
distance is plotted against the distance. If this product were a 
constant, the curves should each be a straight line parallel to the 
horizontal axis. It is seen that between 16 and 60 miles each of 
the three curves is approximately horizontal. Messrs. Duddell 
and Taylor’s measurements will therefore be seen to show that 
the received current from a given constant sending station is 



Trsmsitfissiott distance over perfectly conductive expanse Kilometers 
Fig. 96. Comparison of transmission distances. 

somewhat nearly inversely proportional to the distance. In view 
of the great difficulty of keeping the conditions at the sending 
station constant throughout each of the experiments, and in view 
of the difficulty of measuring the small currents received, Messrs. 
Duddell and Taylor deserve much praise for this laborious and 


PROPAGATION OVER T HE EARTH 


188 


kilometers over a perfectly conductive expanse could be read at 
a distance of 2360 kilometers over the sea; 1450 kilometers over 
fresh water or a rain-soaked soil; 400 kilometers over damp soil, 
and only 70 kilometers over some kinds of very dry soil. Although 
exact quantitative experiments are lacking in regard to the equiva- 
lence of these various distances in a practical case, yet these 
figures do not seem to be very different from the reports of wireless 
telegraph engineers as to the comparative ease of at taining great 
distances over sea and over various kinds of land. 1 

A deduction of the numerical results shown in the above table 
by straightforward reasoning from Maxwell’s theory of electric 
waves, and the agreement of these results with the facts of experi- 
ence, ought to be sufficient to satisfy us that we are dealing with 
true Maxwellian electric waves and not with some new kind of 
electrical manifestation, as some writers have occasionally intimated. 

Absorption Conditioned on Wave Lengths. — In discussing 
Zenneck’s results we have confined our attention to a wave length 
of 600 meters. Zenneck has, however, shown how to modify his 
formulas in order to apply them to other wave lengths; and Pro- 
fessor Fleming has carried the calculations through for several 
other wave lengths, and (haws the following conclusions: 

“ 1. In the case of transmission over sea, the absorption for 
waves of 300 meters wave length is not very large; but, neverthe- 
less, increasing the wave length to 3000 meters is an advantage. 

2. In transmission over laird the absorption of waves 300 meters 
long is very sensible, and increasing the wave length to 3000 meters 
produces a very beneficial effect. 

3. In the case of extremely dry soil the terrestrial absorption 
is very large, anti increasing the wave length from 300 meters to 
3000 meters produces no marked improvement.” 

Effect of Bodies of Water below the Earth’s Surface. — For 
information on this subject the mathematical reader is referred to 
an article by Dr. F. Hack, Annalen der Physik, Vol. 27, p. 43, 1908. 

The Effect of Light and Darkness on Transmission. — Another 
important subject comiected with the long distance transmission 
of wireless telegraph signals is the effect of light and darkness 
on transmission distance. In experiments conducted between 

1 See on this subject, Capt. H. B. Jackson, R.N., F.R.S., “On Some 
Phenomena affecting the Transmission of Electric Waves over the Surface 
of Sea and Earth,” Proc. Roy. Soc. London, 1902, Vol. 70, p. 254. Also 
Fleming, The Principles of Elec. Wave Telegraphy, 1906, p. 606. 



PROPAGATION OVER THE EARTH 


135 


waves, so that these high-frequency currents are given a unidi- 
rectional character and may be measured on a galvanometer by 
reading its deflections, or they may also be measured on a telephone 
receiver by determining what shunt is necessary about the tele- 
phone to reduce its sound to inaudibility. The telephone method 
is the more convenient and this was usually employed by Pickard, 
who, however, reduced his observations to galvanometer readings 



Fra. 97. Observations taken by Mr. Pickard on the relative intensity of 
signals received at different hours of day and night. 

by calibration and by control experiments. The relative intensi- 
ties of received signals , plotted in the diagrams, are the rectified 
currents produced by the electric waves in terms of that rectified 
current which will produce just audible sounds in the telephone. 

We have not yet had a discussion of these crystal rectifiers 
as used to detect or measure electric waves, but it should be said 
in passing that on account of the characteristics of these detectors 
the relative intensities here plotted are not proportional to the 
energy or to the alternating current generated by the received 
signals. We must therefore look upon the intensity values of 
Mr. Pickard’s curves as conditioned by the form of detector used. 
Since, however, the detector employed was one of high sensitive- 
ness and one much used in commercial wireless telegraph 3 r , these 
curves obtained under actual working conditions are highly 
instructive. As a precaution against changes that might occur 
in the detector, Mr. Pickard repeatedly tested the detector by 
throwing it into a circuit containing a constant small alternating 
electromotive force and a galvanometer, and when necessary the 
detector was readjusted so as to give a fixed rectified current 
under the fixed e.m.f. 

By a reference to the curves of Fig. 97 we see that for the partic- 
ular crystal detector, used with a 2000-ohm telephone receiver as in 
actual practice, there was obtained in the telephone receiver about 
30 times as much current near midnight as during the daytime. 




PROPAGATION OVER THE EARTH 


137 


of day and uight transmission of electric waves were first observed, 
the theory was at once advanced that the effect was due to the 
action of the daylight in rendering the air conductive for electricity. 
We have noticed in Chapter II that light, especially ultraviolet 
light, is one of those agencies that ionizes the air by breaking it 
up into charged positive and negative particles, and that air so 
ionized will conduct electricity in a manner known as convection; 
that is, if the ionized air is brought between two plates which are 
charged to different potential, the positively charged particles in 
the ionized air will be driven from the plate of higher potential to 
the plate of lower potential, while the negatively charged particles 
will be driven in the opposite direction. This motion of the charged 
particles constitutes an electric current flowing between the plates. 

Inadequacy of Explanation Based on Conductivity of Air Near 
the Surface of the Earth. — This suggests two ways in which the 
effect of the light would act to decrease the distance of transmis- 
sion by daylight, assuming that the air near the earth is more 
conductive in the daytime than at night. 

(1) The conductivity of the air in the daytime in the neighborhood 
of the sending antenna would cause the charge to leak off the anten- 
na so that it would not be charged to so high a potential and would 
therefore not produce so large an oscillating current as at night. 

(2) The air in the interval between the sending and the receiv- 
ing station, being more conductive in the daytime, would absorb 
more of the energy of the waves than at night. 

Both of these explanations, based on the conduction of the air 
near the earth, seem entirely inadequate to explain the phenome- 
non. The first explanation is untenable because the effects 
of the daylight do not manifest themselves when the stations are 
separated by short distances, and can, therefore , not be localized 
at the sending station. As to the effect of absorption, if we take 
the average experimentally determined value for the conductivity 
of the air near the surface of the earth as 2 X 10~ 2: ’ electromagnetic 
units for a centimeter cube of air, 1 and substitute this value in the 
formula 2 A = A o c ~ 

where for small conductivity 

£= 2 t to* X 3 X 10 10 ; 

1 This value is taken, following Zeimeck, from Gerdien, Physikalische 
Zeitschrift, Vol. 6, p. 647, 1905. 

2 This formula is derived in Boltzmann’s Vorlesungen ueber Maxwells 
Theorie, §96 (Leipzig, 1891). 



PROPAGATION OYER THE EARTH 


139 


regions of the atmosphere than at the surface of the earth, because 
the chief ionizing rays of light are those of very short wave length 
(the ultraviolet), and these short waves of light are strongly 
absorbed by the air, and therefore do not penetrate to a very 
great depth in the earth’s atmosphere. The stratum of upper 
atmosphere, rendered conductive by the sunlight, may serve to 
some extent as a reflector of the electric waves so as to assist in 
confining the waves to the surface of the earth. If this effect 
were appreciable, the waves would be more strongly confined to 
the surface of the earth in the daytime than in the night, and trans- 
mission would be easier in the daytime than at night, except for a 
possible interference between the direct and the reflected wave. 
This interference, if it should exist, would intensify waves of some 
wave lengths and partially annul waves of a different wave length, 
so that by changing the wave length through a range correspond- 
ing to a half period it ought to be possible to turn the interference 
to advantage. No such effects have been found, and the increase 
of the conductivity of the upper air by ionization in daylight when 
looked upon as a reflector does not act in the proper direction to be 
the determining factor in explaining the inequality of transmis- 
sion of electric waves by day and by night. Professor A. E. Ken- 
nelly has called my attention to the fact, however, that there may 
exist in the upper strata, as we pass upward, a gradual change 
from insulating to good conducting strata, which, coupled with 
irregularly distributed conducting areas, might result in a general 
deflection upward of the waves, and a consequent loss of received 
energy, and that tills effect might be greater in daylight than at 
night. This theory lias not yet been given exact mathematical 
expression, so that up to the present we seem not to have found 
an adequate explanation of the difficulties of daytime transmission 
in comparison with night transmission of electric waves to great 
distances. The question is one of great importance from a theo- 
retical standpoint, and if the discovery of the explanation of the 
phenomenon should bring with it the discovery of a means for 
bringing the distance of communication by daytime up to that by 
night, it would remove a very exasperating limitation to electric 
wave telegraphy. 

Experiments with the use of very long electric waves are under 
way by the National Electric Signaling Company and by the 
Marconi Company, and it is reported that some approach toward 
uniformity of day and night transmission has been made. 



ON DETECTORS 


141 


of a natural period of vibration. The following table (Table II) 
taken from Dr. Austin’s paper gives the number of volts required 
to produce just audible sounds in the pair of telephone receivers 
under the application of sinusoidal electromotive forces of various 
numbers of complete cycles per second. 

TABLE II. 

VOLT SENSITIVENESS OF A PAIR OF SCHMIDT-WILKES 800-OHM 

TELEPHONES. 


No. of eyries 
per second. 

Volts 

to produce 
sound. 

audible 

60 

620 

millionths of a volt . 

120 

290 

77 

7 7 17 

180 

170 

77 

7 7 7 7 

300 

60 

77 

7 7 7 7 

420 

17 

77 

7 7 7 7 

540 

8 

79 

77 7 7 

660 

3 

79 

77 7 7 

780 

1.1 

7 7 

7 7 7 7 

900 

0.6 

> » 

7 7 7 1 


Sensitiveness of Galvanometers. — A very sensitive galvano- 
meter of ordinary construction and of about 1000-ohms resistance 
will give a visible deflection with less than one ten-millionth of a 
volt, but such an instrument has loo slow a period (ten seconds) 
to use in indicating wireless telegraph messages. In 1903 Profes- 
sor Einthoven 1 designed a new form of galvanometer that has 
a very rapid period and at the same time a high sensitiveness. 
EinthoveiTs instrument consists of a very fine silvered or platinized 
quartz fiber hung between the poles of a strong magnet. The 
current to be measured is sent through the silver or the platinum 
coating on the fiber, and the fiber tends to move out of the mag- 
netic field. The deflections of this fiber may be observed with a 
microscope', or may be photographed on a rotating drum carrying 
a photographic film. The direction of the deflection of this galva- 
nometer, like that of the ordinary galvanometers, reverses with 
reversal of the current. In one one-hundredth of a second Ein- 
thoven’s instrument will give a deflection sufficiently large to be 
registered on the photographic plate, under application of an e.m.f. 
of one ten-thousandth of a volt. Used in connection with a suitable 


1 Annalen der Physik, Vol. 12, p. 1059, 1903. 




ON DETECTORS 


143 


CLASSIFICATION OF DETECTORS 

We shall describe the detectors under the following more or less 
arbitrary titles: 

Coherers. 

Magnetic Detectors. 

Thermal Detectors. 

Crystal Rectifiers. 

Electrolytic Detectors. 

Vacuum Detectors. 

In illustrating the manner of introducing these various detectors 
into the receiving system a diagram of only a simple form of receiv- 
ing circuit will be exhibited with the descriptions. It is to be 
understood, however, that all the detectors can also be used in 
various forms of direct and inductively connected circuits as well 
as in the simple circuits. 


COHERERS 

As coherers, we shall include only those detectors which employ 
a loose contact and require to be shaken, tapped, or otherwise 
moved to restore the contact to its sensitive condition after the 
receipt of a signal. We have already described the filings-tube 
coherer of Branly and Marconi. A great many modifications of 
this instrument have been made, including the use of a single 
contact or a few contacts in series or parallel, between metallic 
balls or points, to take the place of the filings. Also a great many 
variations in the method of decohering the contacts have been 
made. These will not be described hero. 

These various forms of coherer have their importance in the 
fact that, on the receipt of electric waves, a sufficiently large cur- 
rent is started in the local circuit to operate a relay, ring a bell, 
or give other form of alarm that can be heard at a distance from 
the operator’s desk. Also the current permitted to flow in the 
local circuit of the coherers during the receipt of electric waves is 
sufficiently large to start machinery and control a mechanism (for 
example, a torpedo or dirigible craft) at a distance. This kind of 
result is not easily attained with the other form of detectors listed 
above, which do not permit of the use of sufficiently large currents 
in the local circuit to sound an alarm or start electrical machinery. 



ON DETECTORS 


145 


This is evident from the fact that in some cases the metallic par- 
ticles (e.g., iron or steel) are artificially prepared by oxidizing 
them in order to make of them a good coherer. The poorly 
conductive film may also be present in some cases in the form of a 
sulphide of the metal. On account of the readiness with which 
many metals (called the “ baser metals ”) enter into combination 
with the oxygen or sulphur dioxide of the air, a thin film of oxide 
or sulphide is always present on the surface of most of the baser 
metals, unless special care is taken to remove it. 

Apart, however, from the existence of such films of foreign matter 
at the contact, it seems not impossible that the high resistance 
before the arrival of the waves may be a property of the surfaces 
of even pure metals when these surfaces touch only very lightly. 

If we assume the presence of the poorly conductive film at the 
contacts of the coherer, we may suppose that, on the arrival of 
the electric waves, the poorly conductive film is removed by the 
heat developed by the oscillatory currents. This starts the local 
current, which, developing further heat, still further improves the 
contact and permits the passage of further current. Instead of 
heat being the chief agency in removing tlxe oxide or other poorly 
conductive film, or in bringing together the loose contacts, it may 
be that this is done by the electric attraction between the filings, 
which before the current starts will be charged with opposite signs 
of electricity, and which under the added e.m.f. produced by the 
electric oscillations may attract each other strongly enough to pull 
the contacts together. 

We shall learn more about the electrical properties of high resist- 
ance contacts when we come to the* study of crystal rectifiers . It 
is therefore proposed to omit further discussion of the specific* 
action of the coherers, because of the more* general character of 
the information to be presented later. 

In the meanwhile some of tlu* other detectors which do not 
depend on the properties of a loose* contact are* discussed. 

MAGNETIC DETECTORS 

Rutherford’s Magnetic Detector. — In 1895 and 1896 Pro- 
fessor E. Rutherford 1 discovered a sensitive method of detecting 
electric waves by causing the electric oscillations set up by the 

1 E. Rutherford, “A Magnetic Detector of Electrical Waves and Some of 
Its Applications.” Phil. Trans. Roy. Soc. London, 1897, Vol. 189, A., p.l; 
also Proe. Roy. Soc. London, 1896, Vol. 60, p. 184. 



ON DETECTORS 


147 


band where it approaches and leaves the coils. These n^ignets 
induce magnetic poles in the moving band. One of these induced 
poles, say the South pole, is within the coils, and the two other 



consequent polos (North polos in our illustration) are near the 
point where the bund enters and loaves the (‘oils. 

General Facts in Regard to the Explanation of the Action of 
the Marconi Magnetic Detector. — If we confine our attention 
to a point on the moving band, it is soon that, as the band moves 
forward, this point becomes a North polo outside the coils, changes 
to a South polo wit hin the coils, and becomes again a North pole 
after issuing from the coils. There is, however, within the coils, 
a steady state of magnetization , for although the band is in motion, 
every particle of the band, as it passes a particular point within 
the coils, comes to a particular state of magnetization, so that the 
magnetic condition is fixed with respect to the magnetizing mag- 
nets. This gives a steady state of magnetization within the coils 
and produces no inductive effect in the form of currents in the 
telephone circuit. 

If now a train of electric oscillations passes through the oscilla- 
tion coil b , the magnetization of the part of the band within the 







ON DETECTORS 


149 


to* discover just what is the effect produced on the magnetization 
of the bundle of iron wires by the oscillations within the coil 
surrounding the bundle. A steady current in the coil would 
magnetize the iron wires of the bundle. An oscillatory current, 
according to the experiments of C. Maurain, 1 produces a suppres- 
sion of hysteresis in the iron . 

In explanation of the term “ hysteresis / 9 reference is made to 
Fig. 101, in which magnetizing force is plotted horizontally and 
the magnetization produced by it is plotted vertically. This curve 
represents the hysteresis in a specimen of hard-drawn iron wire 
such as is used in the magnetic detectors. If we start with the 
magnetizing force equal zero, and increase it to OL , the magnetiza- 
tion follows the curve OA. If now we reduce the magnetizing 
force gradually to zero, the magnetization follows the curve AC . 
That is, the state of magnetization produced by the magnetizing 
force when it is decreasing is not the same as the state of magneti- 
zation produced by the force when it is increasing, and after the 
force is removed, some magnetization represented by OC is left 
in the specimen. In order to reduce this magnetization to zero, 
it is necessary to apply a reversed magnetizing force OD . If we 
go on increasing the reversed magnetizing force to OM : the mag- 
netization follows the branch DE of the curve. On decreasing 
and again reversing the force, the magnetization traces out the 
branch EFGA . The complete diagram is called a hysteresis cycle . 

Hysteresis is the property of iron, steel and other magnetizable 
metals characterized by the fact that the change in magnetization 
due to the application of a magnetizing force depends on the pre- 
vious state of magnetization of the specimen. The state of mag- 
netization assumed by a specimen when the magnetizing force is 
gradually removed is not the same as the state of magnetization 
assumed by the specimen when the force is gradually applied. 
The magnetization produced by a given magnetizing force is not 
completely annulled by withdrawing the magnetizing force. The 
hysteresis effect is small in very soft iron, is increased by harden- 
ing the iron, and is very great in glass-hard steel. 

According to the experiments of C. Maurain, which we are now 
discussing in their application to the magnetic detector, the super- 
position of a sufficiently strong oscillatory magnetizing force upon 
a slowly varying magnetizing force causes a suppression of the 
hysteresis in the specimen. If the oscillatory force is weak, the 
1 C. Maurain, Comptes Rendus, Vol. 137, p. 914-916, 1903* 



ON DETECTORS 


151 


South poles and negative under the North poles; and following 
our usual method of plotting, the magnetizing force can be repre- 
sented approximately by the dotted wavy curve H of Fig. 103. 
Now if we suppose the band to be moving in the direction of the 
arrows, the North magnetization under the first South pole will 
not follow the curve of force, but will persist , and follow approxi- 
mately the continuous curve B . If now oscillations produced by 
the electric waves are allowed to flow around the oscillation coil, 
the hysteresis in the band is suppressed, so that the curve of 
magnetization B falls back into the position B' } which is nearer 
the curve of magnetizing force H of Fig. 103. This change from 
the condition B to B l is equivalent to a motion toward the left 
of the magnetic distribution in the coil, and therefore induces 
a current in the coil containing the telephone in circuit. When 
the waves cease, the state of magnetization returns to that repre- 
sented by the curve B. We have thus with each train of waves 
a back and forth shift of magnetization of the band, and conse- 
quently a to and fro motion of the telephone diaphragm. 

While this description of the process seems a very reasonable 
explanation of the action of the detector, yet, for the benefit of 
those readers who may wish a little more insight into the processes 
occurring in iron or steel submitted to an oscillatory field, I beg 
leave to present a brief account of some experiments by E. Made- 
lung, in which he made direct observations of the effect of electric 
oscillation on the magnetization of iron ami steel. 

Experiments of E. Madelung. — A very comprehensive and 
beautiful series of experiments On Magnetization by Rapid Oscilla- 
tions , and on the Operation of the Rulherfonl-Marconi Magnetic 
Detector has been made by E. Madelung, and described in liis 
Gottingen Dissertation. 1 

By means of a very ingeniously devised application of Braun’s 
cathode tube, Madelung was able to obtain on a fluorescent 
screen the hysteresis cycle produced by a slowly varying magnetic 
force, and to obtain also the effect produced on this hysteresis 
cycle by superposing the rapidly oscillating magnetic force pro- 
duced by sending a condenser discharge through the magnetizing 
coils. 

Reference is made to Fig. 104. I. With a slowly varying 
magnetizing force the hysteresis cycle EAKFGE was described. 
II. Upon slowly applying and withdrawing a magnetizing force 
1 E. Madelung; Drude’s Annalen, 1905, Yol. 17, p. 801. 



ON DETECTORS 


153 


A suppression of hysteresis would attain the same end results, but 
instead of being contented with calling the effect “ suppression of 
hysteresis,” which is a purely negative account of the phenomenon, 
Madelung, by his delineation of the spiral course taken by the 
magnetization during the application of the oscillating magnetic 
force, has given us a very distinct picture of the active processes 
occurring in the specimen. He has shown that the magnetic state 
of the iron has been violently agitated by the oscillating mag- 
netic force, and in this way the sluggishness of the specimen in 
following the slowly changing magnetic force has been overcome. 



Fkj. 105. Iligli frequency oscillations superposed on different, parts 

of cycle ( Muddling J. 

Applying this process to our Fig. 103, we must think of the curve 
B us going through a set of vibratory tremors back and forth 
horizontally as it settle* down toward the curve //. These tre- 
mors are of too high frequency to act on the telephone, which 
therefore responds only to the general displacement of the mag- 
netization from the curve B toward the curve //. 

Sensitiveness of the Magnetic Detectors, — The magnetic 
detectors are more sensitive than the coherer, but seem to be less 
sensitive than the electrolytic detector and some of the solid con- 
tact detectors (the crystal detectors). 

THERMAL. DETECTORS 

There are two general classes of detectors in which the heat 
developed by the electric waves is made to manifest itself at the 
receiving station. In one of these classes, including the bolometer 



ON DETECTORS 


155 


this fine platinum wire, which may be as small as one or two 
ten-thousandths of an inch in diameter. In the finished instru- 
ment this fine loop of wire is inclosed in a glass or metal bulb, 
as shown in Fig. 106. The method of using the detector is 
shown in Fig. 107, which contains the detector D in series with 
the antenna A and ground G of a receiving station. In the local 
circuit about the detector is a battery B and a telephone re- 
ceiver T. Oscillations in the antenna circuit passing through 
the detector heat the fine loop of wire. This changes the resist- 
ance of the little loop, and consequently modifies the current in 
the local circuit, and produces a sound in the telephone receiver. 
When the waves cease the little loop rapidly cools, restoring the 
current to its original value. The adaptability of the instrument 
to the receipt of signals is due to the very small heat capacity of 
the fine wire, by reason of which it heats and cools with sufficient 
rapidity to respond with the train-frequency of waves. The diffi- 
culty with the use of this instrument arises in its liability to be 
burned out when the signals become too strong. 

In sensitiveness the barretter falls far below the sensitiveness 
of the electrolytic and crystal detectors to be described later, and 
its use, except for the purposes of laboratory measurements, has 
been practically discontinued. 

Thermoelectric Detectors. — We have already described two 
thermoelectric detectors: Klemeneio’s thermal junction (Chapter 
IX) and Duddeli’s thermogalvanometcr (Chapter XV). These 
instruments change the energy of the electric waves into heat 
localized in a small amount of metal. The heat developed, in the 
ease of Klemencie’s thermal junction, is developed at the thermal 
junction itself; while in Duddell’s instrument the heat developed 
in the “ heater ” is conveyed by radiation and convection to the 
thermal junction. The limiting of the thermal junction produces 
an electromotive force*, which gives rise* to a unidirectional electric 
current in t he local circuit and produce's a galvanometer deflection. 
We have* in these instruments, first, a change of the energy of the 
electric oscillation into heat, and then a change of this heat energy 
again into electric energy. The instruments of Klemencic and 
Duddell, though very useful for the purposes of measurements, are 
not sufficiently rapid or sufficiently sensitive for use in the recep- 
tion of actual messages. 

It has been found, however, that a high resistance contact 
between a common metal and certain crystalline substances, or 



CHAPTER XVII 


ON DETECTORS (Continued ) . — CRYSTAL RECTIFIERS 


We come now to a very sensitive and interesting class of de- 
tectors for receiving the signals of wireless telegraphy and wireless 
telephony. These are the detectors consisting of a self-restoring 
high-resistance contact between solid bodies, and since one of the 
bodies is usually crystalline in character, I have given to this class 
of detectors the name Crystal Rectifiers. 

The crystal rectifiers are self-restoring, and are usually employed 
with a telephone receiver; but a capillary electrometer or galva- 
nometer can be used in the place of the telephone receiver. Many 
of the detectors of this type will give a very strong response without 
a battery in the local circuit, but most of them require the battery 
of small e.m.f. for the best sensitiveness. 

Fig. 108 shows the connections for use of a self-restoring de- 
tector with a battery B in the local circuit. Fig. 109 shows the 



Fig. 108. Crystal contact detector 
with battery in local circuit. 




Gt'/’/A 


75777 



Fig. 109. Contact detector 
without battery. 


detector without a battery. The detector D is shown attached to 
the antenna and ground in a very simple form of receiving circuit. 

157 






DETECTORS — CRYSTAL RECTIFIERS 


159 


Mr. Fahie’s History. 1 In this diagram, C is a carbon pencil 
touching a steel needle N; S is a brass spring by which the pres- 
sure of the contact can be regulated. The adjustment of the 
spring is regulated by means of 
the disc D. 

Professor Hughes used the mi- 
crophone with or without a bat- 
tery in the local circuit ; and 
when the battery was omitted, 
he attributed the sound in the 
telephone to the thermoelectro- 
motive force developed at the Fig. 110 Hughes’s microphonic 
carbon-steel junction. The de- 
tector was more sensitive with a battery in the local circuit than 
without it. 

Various modifications of this microphonic detector of Hughes 
have been employed in practical wireless telegraphy. One modi- 
fication, which had a considerable application a few years ago, 




is obtained by placing a steel needle across two blocks of carbon, 
as shown in Fig. 111. Another is made by placing a granule of 
carbon between metallic plugs in a tube, as shown in Fig. 112. 

The microphone is more sensitive than the filings coherers. 
It is, however, somewhat troublesome on account of sensitiveness 
to mechanical vibrations and on account of liability to cohere 
under strong signals, and it is surpassed in sensitiveness to electric 

1 Fahie, History of Wireless Telegraphy, 1902, Dodd, Mead <fc Co. 






DETECTORS — CRYSTAL RECTIFIERS 


*61 

Pickard’s. Crystal Detectors. — Mr. Greenleaf W. Pickard has 
been very prolific in the discovery of materials of a crystalline 
character that can be used as a member of contact detectors. 
Among the substances used and patented by him in this connec- 
tion are silicon, 1 zincite, 2 chalcopyrite, 3 bornite and molybdenite. 4 

The mounting of Mr. Pickard’s silicon detector, -which is repre- 
sentative of a favorable method of constructing the detectors of 
this class, is shown in Fig. 113- A rod of brass A is pressed down 
by a spring S into contact with a mass of polished silicon B, 


w 


ij 

B 

N 1 
K 


Fid. 113. Pickard’s silicon detector. 

embedded in an easily fusible solder of Wood’s metal, M. The 
solder in which the silicon is embedded is contained in a metallic 
cup P, which rests upon a metallic plate K. Connection to the 
rod A is made by means of the binding post E. Connection to 

1 G. W. Pickard: Electrical World, Vol. 48, p. 1003, 1906; U. S. Patent, 
No. 836,531, filed Aug. 30, 1906, issued Nov. 20, 1906; U. S. Patent, No. 
888,191, filed Nov. 9, 1907, issued May 19, 1908. 

2 G. W. Pickard: U. S. Patent, No. 886,154, filed Sept. 30, 1907, issued 
April 28, 1908. 

2 G. W. Pickard- U. S. Patent, No. 912,726, filed Oct. 15, 1908, issued 
Feb. 16, 1909. 

* G. W. Pickard: TJ. S. Patent, No. 904,222, filed Mch. 11, 1907, issued 
Nov. 17, 190S. 




DETECTORS — CRYSTAL RECTIFIERS 


163 


(1) In determining what currents would flow through the detec- 
tor under a given steady electromotive force; 

(2) In an oscillographic study of the instantaneous values of 
the current through the detector under the action of an alter- 
nating e.m.f.; 

(3) In measuring the thermoelectric properties of some of the 
specimens and comparing the thermoelectromotive force with the 
rectified current. 

Some of the facts obtained in these experiments are presented 
in this and the next chapter. 

Apparatus for Current-voltage Measurements. — Figure 114 
shows a sketch of a form of circuit employed in studying the con- 
ductivity of crystal contact under various conditions, by means of 



Fig. 114. Circuit for studying current-voltage characteristic of 

crystal rectifiers. 

current and voltage measurements. The crystal, held in a clamp, 
is shown at Cr; B is a storage battery; XYZ is a potentiometer 
consisting of two fixed plates of zinc X and Z, and one movable 
plate Y , immersed in a zinc sulphate solution. By means of the 
voltmeter V the difference of potential between the plates Y and 
Z could be read, and the resulting current through the crystal was 
given by a galvanometer or milliammeter at A. The resistance 
of the galvanometer was so small in comparison with the resistance 
of the crystal that the reading of the voltmeter was practically the 
drop of voltage in the crystal. 

The switch S 3 enables the observer to reverse the current in the 
crystal under examination without reversing the galvanometer. 
A known resistance at R could be thrown into circuit with the 
galvanometer for the purpose of calibrating it. 



DETECTORS — CRYSTAL RECTIFIERS 165 

branch II the corresponding values of the current obtained when 
the voltage is reversed. The accompanying table, Table III, 
contains the numerical values from which these curves were 
plotted. 

In the experiment whose result is shown in Fig. 116 and Table III, 
the specimen of carborundum was held in a clamp under a pressure 
of about 500 grams, and it is seen from the table that the current 
in one direction is 100 times as great as the current in the opposite 



Fig. 116 . Curve showing the carborundum contact to be unilaterally 

conductive. 


direction when an electromotive force of 10 volts is applied in the 
two cases. With increase of current through the specimen, the 
ratio of the current in the two opposite directions diminishes. At 
27.5 volts Ci is only 17 times C 2 . 

In this particular experiment the piece of carborundum was sub- 
merged in an oil bath designed to keep the temperature of the 
specimen constant. The piece of carborundum was held in a 
clamp, the jaws of which served to lead the current to the speci- 




DETECTORS — CRYSTAL RECTIFIERS 


167 


curves of Fig* 117 cannot be taken to represent a general oc- 
currence. 

For more details on the effect of pressure reference is made to 
the original publications in the Physical Review . 



Fig. 117. Current- voltage curves of carborundum under different pressures. 


Experiments with Platinized Specimens of Carborundum. — In 

the effort to ascertain what part the form of contact plays in the 
phenomenon of unilateral conductivity in crystals, a number of 
specimens of carborundum were selected with opposite faces plane 
and very approximately parallel, and some of the parallel-faced 
crystals were platinized on one or both of their smooth surfaces 
by the cathode discharge so that they could be put into good 
conducting contact with the electrodes. The metallic surfaces 
thus obtained were in many cases optically plane. 

Platinized on One Face only. — Some of the specimens, plati- 
nized on one face only, gave very remarkable unilateral con- 
ductivity. Table IV shows results obtained with one ol these 
specimens, designated 11 b , when submitted to a pressure of 1 kilo- 
gram. This specimen was .6 mm. thick, with area of about 
1 sq. mm. One of the faces, which was optically true, was heavily 
platinized. The other face was somewhat rough and was without 
platinum. The specimen was held in a clamp with silver jaws. 






DETECTORS — CRYSTAL RECTIFIERS 


169 







DETECTORS — CRYSTAL RECTIFIERS 


371 


values of 60-cycle alternating voltage were applied to the circuit 
containing the detector and galvanometer in series. We shall 
present in a subsequent chapter some oscillograms obtained with 
the crystal rectifiers. Let us, however, first see how a rectifier 
for small alternating currents may be a detector for electric waves. 

RECTIFIERS AS DETECTORS 

Having seen in the preceding paragraphs that certain crystal 
contacts are rectifiers of alternating current, let us now reconcile 
this charaqteristic of the sensitive contacts with their action as a 
detector for electric waves. 

Two Characteristics. — For the purposes of this discussion 1 
we need to fix our attention upon two important characteristics 
of the sensitive contacts above investigated. 

First, the current is not proportional to the voltage; and second, 
the current in the two opposite directions is not the same under 
the same applied voltage. 

A detector may possess one of these characteristics without the 



Fig. 120. Rising current-voltage characteristic (curve A ) and 
falling current-voltage characteristic (curve B). 

other, or may possess both together. A conductor or a combina- 
tion of conductors possessing the first of these characteristics has, 

1 In this we are following very closely the arguments laid down by H. 
Brandes, Elektrotechnische Zeitschrift, Vol. 27, pp. 1015-1017, 1906, and 
Science Abstracts, No. 2078, Vol. 9, 1906. 



DETECTORS — CRYSTAL RECTIFIERS 


173 


an increment of direct current is obtained by the superposition of 
an alternating voltage upon the local direct voltage; that is to say, 
the apparatus is a rectifier. 

In a similar way, it may be shown that if the conductor D has a 
falling characteristic, it also has a rectifying effect, if used with a 
local battery; but in this case the effect of the impressed alternating 
e.m.f is to produce a decrease in the local current. 

Now a crystal contact which is asymmetrically conductive and has 
also a rising characteristic will be a rectifier without a battery and 
also with a suitable battery in the local circuit. Whether it will 
be a better rectifier with or without the battery depends on the 
form of the current- volt age characteristic. 

WHY A RECTIFIER FOR SMALL ALTERNATING CURRENTS ACTS AS A 
DETECTOR FOR ELECTRIC WAVES 

In the preceding sections we have seen that the detectors that 
have certain characteristics are rectifiers for alternating currents. 
In our illustration we applied our alternating e.m.f. directly to the 
circuit containing the detector and the galvanometer, or. telephone, 
in series. But when the detector is used in a wireless telegraph 
receiving circuit, the alternating 
e.m.f. is not so applied, and 
furthermore has a very high fre- 
quency. How is the action of the 
detector to be explained in that 
case ? 

Let us take the case of the 
simple form of receiving circuit 
shown in Fig. 122, with or with- 
out a battery in the telephone 
circuit. 

A train of incoming waves pro- 
duces an alternating e.m.f. in Fiu< l2 2. Detector in antenna 
the antenna circuit. This e.m.f., circuit, 

when in one direction, produces a 

large current through the detector, D , charging the antenna. 
When the e.m.f. reverses, the current from the antenna to the 
ground through the carborundum is smaller, thus leaving the 
antenna charged with a small quantity of electricity. The effect 
of the whole train of waves is additive, so that this charge on the 





CHAPTER XVIII 
ON DETECTORS (Continued) 

FURTHER EXPERIMENTS ON THE CRYSTAL RECTIFIERS 

Having seen in the preceding chapter that the crystal contacts, 
when suitable crystals are employed are detectors for electric waves 
because they are rectifiers for rapid alternating currents, let us 
experimentally investigate the subject a little further. 

Questions Arising in Connection with the Phenomenon. — 
Many interesting questions arise in connection with the phenome- 
non. Is the action localized at the surface of contact between 
the crystal and the metallic electrode ? Is the action due to elec- 
trolytic polarization? Is the action thermoelectric, conditioned 
on unequal heating of the two electrode contacts ? If the phenome- 
non is novel, how is it related to the hitherto studied properties 
of conductors? 

In the experiments on carborundum, performed by the writer 
and partially presented in the preceding chapter, the investigation 
of these questions met with limitations on account of the form of 
occurrence of the carborundum in discrete masses to which elec- 
trodes could not be rigidly attached, so that the conditions at the 
electrodes could not be widely varied. However, by increasing 
the pressure of the electrodes against the carborundum beyond a 
certain limit, and by cathodically platinizing the surfaces of the 
carborundum at both the contact areas, we have seen that the 
rectification, though not entirely eliminated, was rendered very 
imperfect; that is to say, the ratio of the strength of the current in 
one direction to that in the reverse direction approached unity. 
On the other hand, platinizing one only of the surfaces of contact, 
while the other surface was left unplatinized, generally rendered 
the rectification more nearly perfect. This fact indicated that the 
seat of the action was the area of contact with the electrodes, and 
that the action at the two contacts were usually in opposition to 
each other, so that when the action at one of the contacts was 
reduced by platinizing, the rectification at the other contact 
appeared more pronounced. 


175 



DETECTORS — CRYSTAL RECTIFIERS 


177 


action, it must be of such a character as to change the nature of the 
electrodes or of the crystal only very slowly, if at all. 

On the Question of a Possible Thermoelectric Origin of the 
Phenomenon. — It is apparent that the disposition of the crystal, 
with a high-resistance contact of a metal against it at one side and 
usually a comparatively low-resistance contact at the other side, 
is exactly the most favorable for the development of heat at the 
high resistance junction. This heat being localized at a very 
small area, would raise the temperature of that area considerably. 
Now when the junction of two dissimilar conductors (e.g., bismuth 
and antimony) is heated, an electromotive force is developed at 
the junction. And for all we know, unless we try it, the contact 
of the crystal with the metal may have an enormously higher 
thermoelectromotive force developed than that developed at pre- 
viously known thermal junctions. 

If this is true, then when the current is in one direction the 
thermoelectromotive force would add to the applied voltage and 
produce an excessive current, while with the current in the opposite 
direction the thermoelectromotive force would subtract from the 
applied voltage and produce only a small current. This explana- 
tion of the phenomenon seems at first alluringly simple, and has 
been adopted by a number of writers and inventors, some of whom 
have, however, afterwards changed their views. But many per- 
sons still hold to the idea that these crystal-contact detectors are 
thermoelectric detectors, and they are so described in many trade 
catalogues, especially in Europe. 

In fact, there is so much genuine circumstantial evidence in 
support of the thermoelectric hypothesis, that it seems very im- 
portant to present with some thoroughness the experimental facts* 
that exclude this hypothesis. 

Extension of the Experiments to Other Crystals. — In order to 
carry out such an investigation a search was made for other 
crystals showing properties similar to carborundum but occurring 
in a form more suitable for study. After anatase and brookite 
and molybdenite had been discovered to be rectifiers and had been 
tested, it was found that the required conditions were best full- 
filled by molybdenite. 

I shall therefore describe the molybdenite detector. I shall 
then show and describe some oscillograms of alternating current 
through several crystal detectors, and shall afterwards return to 
some thermoelectric experiments. 



DETECTORS — CRYSTAL RECTIFIERS 179 

shoulder of the cap, with the upper surface of the molybdenite 
exposed above. At the free surface of the molybdenite contact 
is made 1 with the metallic rod P. 

The rod P was either supported unadjustably, as in the author’s 
experiments on sound, or it was mounted in a manner to permit 
of ready adjustment, as is shown in Fig. 124. The clamp K 
containing the molybdenite is metallically connected with the 
binding post H (Fig. 124). Another binding post is attached 



Fig. 124. Mounting for molybdenite. 


to the metallic block A, on top of which is supported a stout 
spring B . Through a hole in B provided with a set-screw, the 
rod P is allowed to drop down into contact with the surface of 
the molybdenite at K . The set-screw is then tightened against 
P, and the final adjustment is made by the slow-motion screw S. 
The apparatus is connected in circuit by means of the binding 
posts, so that the current of the circuit is made to enter the molyb- 
denite through the contact area between P and the molybdenite 
and leave by way of the contact between the molybdenite and the 
cap C 7 or the reverse. It is found that a much larger current 
flows in one direction than in the reverse direction for a given 
applied electromotive force. 

The current-voltage curves (see Figs. 125, 126 and 127 ) resemble 
those of the carborundum detector, but large rectified currents 

1 In the diagrams of Fig. 123 and Fig. 124 the lower end of the rod P is 
shown pointed. It is found, however, that the end of the rod P may be blunt 
or even flat with an area as great as 4 sq. mm. without much loss of sensitive- 
ness of the instrument as a receiver for electric waves or as a rectifier. 



DETECTORS — CRYSTAL RECTIFIERS 


181 


are obtained with very small voltages in the case of the molyb- 
denite, which characterized the molybdenite rectifier as much 
more sensitive than the carborundum as a detector for electric 
waves. 



OSCILLOGRAPHIC STUDY OF CRYSTAL RECTIFIERS 

An oscillogram is a photograph showing the rapidly changing 
values of the current in a circuit when a rapidly changing voltage 
is applied to it. In the case of the crystal rectifiers a current of 
only a few thousandths of an ampere could be sent through the 
crystal contact without destroying its rectifying power. It was 
therefore necessary to employ a very sensitive apparatus, — one 
that would deflect with these small values of the current, and 
would reverse when the current reversed, and that at the same time 
would be so rapid in its action as not to show any appreciable lag 
when the current through it was rapidly changing. The purpose 
of the experiment was to see if the current changes in the detectors 
followed the voltage changes at once or if they lagged behind, as 
would be the case if the action of the detector depended on heating 
or cooling, because heating and cooling require time. Also, if 
electrolytic action entered into the phenomenon it ought to show 
in the oscillograms. 

After much experimenting the necessary sensitiveness of appa- 
ratus was finally obtained with a Braun's cathode tubeoscillograph. 
This apparatus makes use of the fact that when a high electro- 
motive force, say 20,000 volts, is applied to two aluminum elec- 
trodes sealed into a glass tube, from which the air is pumped to 



DETECTORS - CRYSTAL RECTIFIERS 


183 


could be made, during which time the image of the spot moved 
over the sensitive film 4800 times, without any failure of per- 
fect superposition, and without any appreciable fogging of the 
film. 

The deflecting electromagnets MM had a combined resistance 
of 436 ohms, and were provided with soft iron cores about 6 milli- 
meters in diameter. With these deflecting coils a direct current 
of 1.5 milliamperes gave a deflection of 1 cm. on a ground glass put 
in the place of the sensitive film at the back'of the camera. A 
calibration for different values of direct current through the coils 
showed the deflections of the light spot to be proportional to the 
current, for the small values of the current employed, and showed 
no evidence of hysteresis in the iron. 

The Oscillographic Photographs. — Reproductions (reduced to 

of a characteristic set of the photographs obtained with a 60- 
cycle alternating e.m.f. are given in Plate I. Oscillograph No. 1 
was taken with the molybdenite rectifier adjusted to give practi- 
cally perfect rectification. No. 2 is with the same rectifier slightly 
out of adjustment (overloaded), so that the rectification is less 
perfect. No. 3 is with the same rectifier further out of adjustment. 
No. 4 is an oscillographic record with the carborundum rectifier. 
No. 5 is with the rectifier of brookite. In taking No. 2 the rectifier 
was submerged in oil, to test the effect of cooling. 

Three Exposures. — In making these pictures the following 
steps were taken: The drum carrying the film was set rotating. 
The high-potential current obtained from Professor Trowbridge’s 
40,000 volt storage battery was started in the tube. The potential 
V (Fig. 128) and the contact of the rectifier were adjusted so that 
the deflection of the luminescent spot on the fluorescent screen 
showed good rectification. Exposure of about 2 minutes was then 
made. This exposure gave the heavy line of the oscillograms. 

The switch at T (Fig. 128) was then thrown open, so that no 
current was flowing in the electromagnets and the luminescent spot 
came to its zero position. The exposure in this position was made 
for a shorter time of about 40 seconds. This traced a thin straight 
line along the centre of the picture and gave the axis of zero 
current. 

The switch at T was then thrown to the position to put the resist- 
ance R in the circuit in place of the crystal. The resistance R 
had been previously adjusted, so that the amplitude of the deflec- 
tion with R in the circuit should be equal to the maximum am- 



DETECTORS — CRYSTAL RECTIFIERS 


185 


plitude with the crystal in the circuit. With the resistance R in 
circuit an exposure of about 1 minute was made, giving the light 
sinusoidal curve of the picture. 

On each picture the three exposures give, therefore, (1) the form 
of the rectified cycle as a heavy line, (2) the position of the axis 
of zero current, as a straight line through the figure, and (3) the 
form and position of the alternating current cycle when an 


TABLE VI 

TABULAR DESCRIPTION OF THE OSCILLOGRAPHIC RECORDS OF PLATE I 





Maximum 


Equiva- 
lent Re- 

No. 

Material of Rectifier. 

Condition. 

Rectified 
Current in 

R. M. S. 
Alternat- 




Milliam- 

ing Volts. 

sistance 
in Ohms. 





peres. 



1 

Molybdenite j 

Good adjust- 
ment 


■ 4.9 

3.54 

400 


r 

Out of best ad- 







justment, 





2 

” 4 

1 

submerged in 
oil and over- 


4.9 

3.54 

400 


{ 

i loaded 

J 




3 

{ 

Out of best ad- 
justment 

1 

J 

j- 4.5 



4 

Carborundum plat- 
inized on one side 

Overloaded 


5.4 

22.0 

6000 

5 

Brookite 

>> 

! 

3.0 

2.22 ; 

; 1 

992 


equivalent resistance R is substituted for the rectifier. The last 
named cycle appears in the pictures as a thin-lined sine curve. 
This curve is in phase with the impressed voltage immediately 
about the crystal, and is referred to below as the “ voltage-phase 
curve.” 

Coordinates. — In tracing all the curves, the motion of the light 
spot over the paper is from left to right; the time coordinate is, 
therefore, horizontal and is drawn as usual from left to right. 

The scale drawn in ink at the left-hand margin of each picture 
gives the value of the current, one division being one milliampere. 

Conditions. — A tabular description of the conditions under 
which each of the records was taken is contained in Table VI. 

A discussion of the records follows: 

Oscillogram Nos- i, 2, and 3 — Molybdenite. — The pressure 




DETECTORS — CRYSTAL RECTIFIERS 


187 


differs from the molybdenite cycle in the absence of a lead at the 
negative maximum and at the point of rising from the zero axis. 
This anomaly in the case of the carborundum rectifier is seen 
later to be the effect of its high resistance. 

Oscillogram No. 5 — Brookite. — The form of the cycle ob- 
tained in this case is intermediate between the carborundum cycle 
and the cycle of oscillogram No. 3. This is consistent with the 
value of its resistance. 

In order to investigate the meaning of the lead of the rectified 
cycles in the several cases, the oscillograms had to be examined 
mathematically with the aid of the theory of alternating currents. 

Only the conclusions from this mathematical examination are 
here given. The mathematical reader is referred to the original 
paper. 1 

Conclusions from an Examination of the Rectified Cycle with 
the Aid of Alternating Current Theory. — (1) The case of the 
advance of the rectified cycle on rising from the axis of no current 
is shown in the mathematical discussion, above referred to, to 
be due to the fact that after a dormant half-period the current in 
the circuit follows the ordinary exponential “ building-up ’ ’ curve 
for a time before coming into coincidence with the sine curve. 
This building-up curve starts from the axis with zero lag, and is, 
therefore, in advance of the sine curve. It is chiefly due to the 
self-inductance in the oscillographic circuits. To this effect of 
self-inductance is to be added the effect due to the higher resist- 
ance of the rectifier for small currents than for large currents. 
This higher resistance brings the building-up curve a little nearer 
to the sine curve. 

(2) The slightly quicker descent of the rectified cycle on ap- 
proaching the axis after having traversed the upper half of the 
curve is also due to this higher resistance of the rectifier when 
traversed by smaller currents. 

(3) The very significant lead of the negative maximum ahead of 
the corresponding voltage-phase maximum is explicable on the 
assumption that the rectifier has a much higher resistance in the 
negative direction than in the positive direction. We have shown 
in the mathematical discussion that the angle of lag of the voltage- 
phase cycle behind the impressed voltage, determined by the 


1 G. W. Pierce: Physical Review, 1909, Vol. 28, p. 153; or Proc. Am. Acad, 
of Arts and Sciences, 1909, Vol. 45, p. 317. 



DETECTORS — CRYSTAL RECTIFIERS 


189 


the result obtained in an oscillographic study of the electrolytic 
detector, where an integrative action was discovered (see next 
chapter). 


THERMOELECTRIC PROPERTIES OF MOLYBDENITE 

In the present section an account is given of the investigation of 
the thermoelectromotive force of molybdenite against copper and 
a determination of the temperature coefficient of resistance of 
molybdenite. Apart from their possible bearing on the action 
of the rectifier, the thermoelectric properties of molybdenite are of 
interest in themselves. 

Thermoelectromotive Force. — Five specimens were mounted 
for the study of the thermoelectromotive force of molybdenite 
against copper. These specimens are referred to as “ A ,” “ B,” 
“ C,” “ D,” and “ E.” The method of mounting the specimen E 
is shown in Fig. 129. A thin sheet of molybdenite .1 or .2 mm. 



Fig. 129. Apparatus for studying thermoelectric properties 
of molybdenite. 

thick, 2 cm. wide, and 8 cm. long, was cemented between two glass 
microscope slides G with a cement made of water-glass and calcium 
carbonate. The molybdenite was then copper-plated over a small 
area at each of the exposed ends MM, and to these copper-plated 
areas were soldered copper wires .2 mm . in diameter, so as to form 



RECTIFIERS 


191 




DETECTORS — CRYSTAL RECTIFIERS 


193 


other. This preliminary test proved very interesting in that it showed 
that one may find all over many of the pieces out from a crystal of 
molybdenite points where the substance is thermoelectrically positive 
and other points where it is thermoelectrically negative. These posi- 
tive and negative points sometimes lie so near together that with a 
fine-pointed exploring electrode attached to a galvanometer and 
warmed by heat conducted from the hand, one may find the deflec- 
tions of the galvanometer reversed from large positive values to 
large negative values on making the slightest possible motion of 
the pointer over the crystal. 

Explorations of this kind failed to show any definite orientation 
of the thermoelectric quality with respect to the crystallographic 
axes. 

The existence of small thermoelectrically positive and negative 
patches in a piece of the molybdenite may indicate that the ther- 
moelectromotive force measured by attaching wires to the speci- 
men is too low on account of the inclusion under the electrodes of 
both positive and negative areas which would partially neutralize 
the thermoelectric action against another electrode. 


TABLE IX 



Thermoelect romotive Force in Mi- 



crovolts, per Degree Centigrade, 


Substance. 

at 20° C. 

Authority. 


Against Copper. 

Against Lead. 


Molybdenite A . . . . 

110 

113 

Present experiment 

” B . . . 

— 230 

-227 

■> i 

” C . . . 

175 

178 

’’ 

” 1) 

-415 

-413 

’ 1 

E... 

— 720 

-717 

’ 1 

Silicon 


-400 

F ranees G . W ic k 1 

Bismuth 


- 89 

Matthiessen 1 

Antimonv 


26 

’ ’ 

Tellurium 


502 


Selenium 

t 

; i 

S07 



1 Phys. Rev., 25, 390. 2 Everett, Units and Physical Constants. 


It may be said in passing that the specimens D and E , with 
soldered corinections , still showed the phenomenon of rectification 
when used with alternating currents, even when the two junctions 
of the copper with the molybdenite were in oil baths at the same 




DETECTORS — CRYSTAL RECTIFIERS 


195 


20° the decrease of resistance per degree increase of temperature 
is 1.19 percent. 

Plausibility of Thermoelectric Explanation. — The large thermo- 
electromotive force of the molybdenite against the common metals, 
together with its large negative temperature coefficient of resist- 
ance, lends plausibility to the hypothesis that the rectification 
is due to thermoelectricity. For if we pass an electric current 
through the rectifier and the current begins to make its way 



Temperature 

Fig. 132. Resistance and conductance of molybdenite as a function 

of the temperature. 

through a small area at the contact, this small area is heated and 
decreases in resistance, so that the greater part of the current flows 
through this particular small area, heating it still more, while the 
portions of the contact through which the current has not started 
remain cool and continue to offer a high resistance. The effect 
of this action is to confine the heating to an extremely small area, 
which is the condition necessary for the extreme^" rapid and 
efficient action of the rectifier, on the hypothesis of a thermo- 
electric explanation. That there is, however, an insuperable ob- 


DETECTORS — CRYSTAL RECTIFIERS 


197 


denite or the circuit through the constantan could be read .on the 
galvanometers A or G. Also the rectified current obtained by 
applying the alternating voltage V could be read on the galva- 
nometer A . When the thermal current or the rectified current 
through A is in the direction of the arrow B , the molybdenite, 
following the usage in thermoelectricity, is said to be positive . 
When the current in A is in the direction opposite to the arrow B, 
the molybdenite is said to be negative . 

The results obtained with a number of specimens of molybdenite 
when heat was applied above , and when heat was applied below, 
and when the alternating voltage was applied, are contained in 
Table X. 

TABLE X 

SIGN OF MOLYBDENITE WHEN HEATED ABOVE OR BELOW 
AND* WHEN SUBJECTED TO ALTERNATING VOLTAGE 


Specimen No. 

Heated Above. 

Heated Below. 

Under Alternat- 
ing Voltage. 

75 

+ 

» 

_ 

81 

+ 

— 


Turned over 

-h 


— 

93 

— 

4- 

4- 

Another point 

- 

— 

4- 

J T 

Turned over 

— 

— 

+ 

— 

— 

4~ 

78 

+ 

4- 

4* 

Another point 

4- 

— 

— 

a 

+ 

4- 

— 

94 

— 

— 

4- 

Another point 

— 


+■ 

j i 


+ 

4- 


From this table it appears that the thermoelectric voltage when 
the junction is heated by heat conducted from above, in twelve out 
of the thirteen eases tried, is opposite to the direct voltage ob- 
tained when an alternating current is passed through the junction. 
When the heat is conducted to the junction from below , through the 
molybdenite , the thermoelectromotive force in four cases is opposite 
to the rectified voltage, and in nine cases is in the same direction 
as the rectified voltage. In only one case, one point of No. 78, 
is the rectified voltage in the same direction as the thermal voltage, 
both when the junction is heated from above and when it is 
heated from below. 

In all of these cases the heat was applied in the neighborhood of 
the same junction, and there is no opportunity for heat to get to 




DETECTORS — CRYSTAL REC'fjEDERS 


C499 


When, on the other hand, as a control expenm&to, i^e^Kszras applied 
to the copper-molybdenite junction from below 
conducted through the molybdenite and through the - 

denite junction to get to the copper-constantan junction^ the 
heating shown by the auxiliary copper-constantan junction was 
11.4° C., while the thermal current from the copper-molybdenite 
junction was only .2 microamperes. In both the case of the recti- 
fied current and the case of the application of heat from below 
the heat had to be conducted from the point of rectification to the 
auxiliary junction. Therefore, with a rise of temperature of the 
auxiliary junction 1100 times as great as the rise shown during 
the rectification, the thermal current in the copper-molybdenite 
circuit was of the rectified current; that is to say, the rectified 
current, for a rise of temperature of xi^ of a degree of the auxiliary 
junction (being approximately a linear function of the tempera- 
ture) was less than zvihroTr of the rectified current from an alter- 
nating current producing the same rise of temperature. 

Summary of Conclusions from the Experiments with the 
Crystal Rectifiers. — 1. An examination of the characteristics of 
contact detectors using carborundum, anatase, brookite, hessite, 
iron pyrites, and silicon shows that we are dealing with the same 
kind of phenomenon in the case of all these crystal substances. 
The various other crystal-contact detectors which I have not 
examined probably act in the same way. 

2. At the contact between the crystal and a common metal, 
or between two different crystals, or between two apparently simi- 
lar crystals, there is asymmetric conductivity, permitting a much 
greater current to flow in one direction than in the other under 
the same applied voltage. 

3. These contacts all have a rising current-voltage charac- 
teristic. 

4. These crystals all have a large thermoelectromotive force 
against the common metals, and the amount and the direction of 
this thermoelectromotive force is different at different points on 
the crystalline bodies. 

5. The rectifying effect is also different in amount and direction 
at different points of the crystalline body; the direction of the 
rectifying effect is often opposite to the effect that would be 
obtained by heating the contact. 

6. Thermoelectricity does not explain the phenomenon of rec- 
tification, but the two effects, since both exist in such marked 



CHAPTER XIX 


OK DETECTORS ( Concluded ) 

THE ELECTROLYTIC DETECTOR, AND VACUUM DETECTORS 

Description of the Electrolytic Detector, — The electrolytic 
detector for electric waves, as described by Fessenden 1 and shortly 
after by Schloemilch, 2 * consists of a cell containing an electrolyte 
and having one electrode of very small area, usually in the form of 
an extremely fine wire of platinum, and as the other electrode a 
larger area of platinum or some other metal. When used in wire- 
less telegraphy the two electrodes are connected in a circuit upon 
which the electric oscillations are impressed, so that the rapidly 
oscillating electric currents in the circuit are made to traverse the 
cell of the detector. An example of a simple form of receiving 
circuit, with the detector connected in the 
antenna, is shown at MDG of Fig. 134. A 
local circuit TED , through the detector, con- 
tains a telephone receiver T and an adjustable 4 
source of e.m.f., which is used to polarize the 
detector by sending through it and the tele- 
phone a small direct current. Under the 
action of the electric oscillations through the 4 
detector the current in the telephone receiver 
is modified so as to produce a sound in the 
telephone with a period determined by the 
train frequency of the incident electric waves. 

The action is localized at the contact of the Fn? 134 Circuit with 

electrolytic detector. 

fine wire with the electrolyte. 

Details of the Electrolytic Detector. — The electrolyte employed 
in the electrolytic detector is usually 20% nitric acid, though 
almost any electrolytically conductive liquid (e.g., dilute sulphuric 
acid, common salt solution, caustic soda, etc.) may be used. For 
a highly sensitive detector the fine platinum wire employed as 

1 Fessenden, TJ. S. Patent, No. 727,331, filed April 9, 1903; issued May 5, 
1903. 

2 Schloemilch, Elektrotechnische Zeitschrift, Vol. 24, p. 959, Nov. 19, 1903* 

201 





ELECTROLYTIC AND VACUUM DETECTORS 


203 


This makes the detector itself a primary battery. 

This arrangement for which a United States patent has been 
issued to Schloemilch, 1 and also to Shoemaker, 2 would seem to be 
incapable of the high sensitiveness attained by the form in which 
the accurately adjustable external voltage, as in Fig. 134, is 
employed. 

Regarding the Theory of the Electrolytic Detector. — Con- 
siderable diversity of opinion has been expressed by various writers 
as to the manner in which the electrolytic detector acts as a 
receiver for electric waves. Professor Fessenden in his original 
patent attributes the action to heat, and he calls this form of 
detector a “liquid barretter.” Pro- 
fessor Armagnat, 3 who has made 
an experimental study of the sub- 
ject, attributes the action to a 
rectifying effect resulting from 
polarization. Armagnat obtained 
a curve of the form of Fig. 137 
for the current-voltage character- 
istic of the electrolytic detector. 

Dr. L. W. Austin 4 also found that 
the electrolytic detector acted as 
a rectifier for small alternating 
currents, but came to the opinion 
that heat, chemical action, rectifi- 
cation, and electrostatic attraction * Ui - 1 ^'- | ( . urrent -voltage curve 
J of electrolytic detector. 

across the gas film might have a 

part in the explanation of the phenomenon when the detector was 
used with electric waves. 

A doubt that arose in the minds of some investigators of the 
subject as to a possible explanation of the phenomenon in terms 
of rectification alone came, it seems, from the idea that there 
could not be energy enough in the electric waves received at great 
distances to produce the effects in any other way than by a 
triggering action, by which the local energy of the battery was 

1 Wilhelm Schloemilch, U. S. Patent, No. 936,258, filed Oct. 3, 1903, Issued 
Oct. 5, 1909. 

2 Harry Shoemaker, U. S. Patent, No. 795,312, filed Feb. 13, 1905, issued 
July 25, 1905. 

3 Armagnat, Bui. soc. framjaise, session of April, 1906, p. 205 \ Journal de 
Physique, Vol. 5, p. 748, 1906. 

4 Austin, Bui. Bureau of Standards, Vol. 2, p. 261, 1906. 




ELECTROLYTIC AND VACUUM DETECTORS 


205 


This quotation shows that Pup in had employed the electrolytic 
detector in 1899 as a rectifier for electric waves of Hertzian fre- 
quency, and that he had a well-defined explanation of the processes 
occurring in the rectifier. I have made some experiments that 
fall into close agreement with Pupin’s explanation of the phenome- 
non. These are described in the succeeding paragraphs. 

OSCILLOGRAPHIC STUDY OF THE ELECTROLYTIC DETECTOR 1 
In these experiments the current through the detector under the 
action of an alternating e.m.f., superposed on a polarizing current, 
is determined by means of an oscillograph. The application of 
the oscillograph to the problem gives the instantaneous values of 
the current through the detector, and permits an examination 
of the wave form of the rectified cycle. The oscillographic 
apparatus was the Braun’s tube described in Chapter XVIII. 

Circuits Employed with the Detector in Taking the Oscillo- 
grams.. — The electrolytic detector used in these experiments made 



Fig. 13$. Oscillographic* apparatus and circuits for study of electrolytic 

detector. 

use of a platinum point, .0002 inch in diameter, dipping into 20 
per cent nitric acid, and was adjusted to high sensitiveness as an 
electric wave detector immediately before taking the oscillograms. 
A diagram of the circuits employed in the experiment, together 
with a sketch of the oscillographic apparatus, is shown in Fig. 138. 

1 This account is an abridgment of an article by the author on 4 ‘The 
Electrolytic Detector, Studied with the Aid of an Oscillograph.” Physical 
Review, 1909, Vol. 28, p. 56. 




ELECTROLYTIC AND VACUUM DETECTORS 


207 


about the detector. 1 A similar curve was made use of in the 
experiments of the preceding chapter and is there discussed. In 
the present experiments, because of the employment of the polar- 
izing current with the rectifier, a question arises as to the appro- 
priate method of taking this cycle. Two different methods were 
tried, either of which, by proper elimination of the constants of the 
oscillographic apparatus, will give the desired result. The method 
yielding simplest results for the voltage-phase cycle is the following: 
After the exposure for the rectified cycle had been made, the alter- 
nating voltage was left unchanged, and a resistance was substituted 
for the rectifier. A double adjustment of the substituted resist- 
ance and the direct voltage was made by successive approxima- 
tions until the result was attained that (1) the direct voltage alone 
gave through the substituted resistance a current equal to that 
used in polarizing the rectifier and (2) the alternating voltage 
superposed on this direct current gave a deflection of the lumi- 
nescent spot to a point coincident with the maximum point attained 
with the rectifier in the circuit. This means that the voltage- 
phase cycle was taken with the axis of polarizing current as axis, 
and with amplitude equal to the maximum amplitude of the recti- 
fied cycle. This method was employed in oscillograms 1, 2, and 5. 

The second method of taking the voltage-phase cycle was as 
follows: The polarizing voltage was reduced to zero, the detector 
was short-circuited, and an alternating voltage equal to that used 
with the detector was applied to the circuit. This method was 
employed in oscillograms 3 and 4. 

Coordinates of the Oscillographic Curves. — In taking all of 
the curves of the oscillograms, the motion of the light spot over 
the film is from left to right; the time coordinate is, therefore, the 
horizontal scale of the curves and is drawn as usual from left to 
right. The current coordinate is given in the scale drawn in ink 
at the left-hand margin of each picture — one division being one 
milliampere. 

DISCUSSION OF THE OSCILLOGRAMS OF PLATE II 

The oscillograms shown in Plate II are reproductions of positives 
printed from the films carried by the rotating drum. They were 
taken with a 60-cycle alternating current applied to the circuit 

1 The ordinary method, which would be to take the leads from the two sides 
of the detector through a high resistance to the oscillograph, could not be used 
because the oscillograph was working at the limit of its sensitiveness on the 
full voltage without the added resistance. 



ELECTROLYTIC AND VACUUM DETECTORS 


209 


containing the electrolytic detector. The reproduction is one- 
third the size of the original. The several curves shown in the 
plate were obtained with different polarizing currents superposed 
on the circuit. Table XI contains a tabulation of the polarizing 
current and voltage, the applied alternating voltage, the maximum 
current through the detector, and the substituted resistance 
employed in taking the voltage curve. 


TABLE XI 

TABULAR DESCRIPTION OF THE OSCILLOGRAPHIC RECORDS 


No. 

Polarizing Direct 
Current in Milli- 
amperes. 

Polarizing E.M.F. 
in Volts. 

R.M.S. 
Volts A.C. 

Maximum 
Positive Cur- 
rent through 
Detector in 
Milliamperes. 

Equivalent 
Resistance 
in Ohms. 

I 1 

.1 

1.45 

2.09 

2.37 

440 1 

2 

1.0 

5.5 

4.00 

9.6 " 

70 

3 2 

1.2 

5.5 

4.00 

9.6 

00 2 

4 2 

1.4 

Not measured 

5.00 

10.0 

OO 2 

5 

2.2 

) > 

5.00 

11.0 

150 


1 It should be noticed that the sensitiveness of the oscillograph when No. 1 
was taken was three times as great as when the other oscillograms of the plate 
were taken. 

2 The voltage-phase cycle of oscillograms 3 and 4 was taken with the polar- 
izing current omitted, so that they have the axis of no current as axis of the 
cycle. 

Point Anode or Cathode — the Large Loop in the Direction of 
the Polarizing Current. — Some of the oscillograms were taken 
with the polarizing current from the point to the electrolyte and 
some with the polarizing current in the opposite direction. Al- 
though the values of the polarizing voltage required to produce a 
given polarizing current were different in the two cases the general 
characteristics of the cycle were the same. A reversal of the polar- 
izing current reversed the rectified current, and whether the polar- 
izing current was from the point to electrolyte or in the opposite 
direction the large loop of the rectified cycle (always oscillographed 
positively) was obtained when the alternating current was flowing 
in the same direction as the polarizing current. 

The Form of the Rectified Cycle. — The cycle obtained with the 
rectifier in the circuit has the same general form in all the pictures. 
When the current, having traversed the positive loop, comes to 
the axis of zero current, it follows along this axis for a short way, 




ELECTROLYTIC AND VACUUM DETECTORS 


211 


region to the immediate right of the negative maximum . This 
rise is more striking in the original photographs than in the repro- 
ductions; and, though small, it deserves attention, because the 
occurrence of this small positive maximum is evidence of the 
existence for about rsW of a second of a positive e.m.f. 
greater than the e.m.f. i m mediately following. Now in this 
part of the cycle the externally applied e.m.f. is greater follow- 
ing the rise than during the rise; therefore the rise indicates the 
existence of a positive e.m.f. in the circuit itself. This is capable 
of the following explanation in terms of the theory of polarization. 
After the prevalent external e.m.f. has been in a negative direc- 
tion and has returned to zero, the polarization tension which has 
been opposing the negative current at the electrode continues to 
exist for a short time and produces a positive current. This 
action, resembling that of a capacity, is familiarly known as the 
polarization capacity of the electrode. By the existence of the 
small positive maximum near the axis of the cycle, the oscillogram 
shows that the polarization capacity of the electrode is not entirely 
negligible. Evidence of the existence of this polarization capacity 
is clearly given by the oscillograms 1, 2, and 3. The oscillograms 
4 and 5, while not having a positive maximum near the axis, show 
also a striking tendency toward a maximum at this point, which is, 
however, masked by the rapid rise of the building-up curve in this 
part of the cycle. 

CONCLUSIONS IN REGARD TO THE ELECTROLYTIC DETECTOR 

1. The whole phenomenon of the rectification of small alter- 
nating currents by the electrolytic detector seems to be explicable 
in terms of the theory of electrolytic polarization. 

2. The polarization capacity of the small platinum electrode is 
not entirely negligible, even with currents making only 60 cycles 
per second. The polarization capacity may, however, aid in pro- 
ducing a rectified current as well as in opposing this effect, and 
apart from the effect of this capacity on the tuning of the circuit . 
does not detract from the utility of the rectifier as a detector for 
electric waves. 

3. The present conclusions in regard to the action of the detector 
are entirely in accord with Pupin's original brief description of the 
phenomenon as quoted above. 



ELECTROLYTIC AND VACUUM DETECTORS 


213 


current through it, negative electrons are sent off from it and render 
the space between the filament and the cylinder conductive for 
an electric current, provided the e.m.f. producing this current is 
directed from the cylinder to the hot filament. In case the 
e.mJ. is applied in the opposite direction, no current, or a much 
smaller current, flows. An oscillating e.m.f. applied to the cylin- 
der and filament produces more current in one direction than 
in the opposite direction. 

One method of connecting the valve into a wireless telegraph 
receiving circuit is shown in the diagram, which is taken from 



Fig. 140. Professor Fleming Fig. 141. Circuit employed 

vacuum tube rectifier. by Dr. DeForest with vacuum 

detector. 


Professor Fleming’s U. S. Patent Specifications. Here the valve 
is in a circuit connected inductively with a wireless telegraph 
antenna. Electrical oscillations in the antenna induce an oscil- 
lating electromotive force in the coil l\ and this oscillating e.m.f. 
sends more current in one direction than in the opposite direction 
through the valve and through the current -indicating instrument l. 

A modification of the method of connecting the indicating 
instrument to the oscillation valve has been made by DeForest 
so as to permit the use of a telephone as indicator. A diagram of 


CHAPTER XX 


ELECTRICAL RESONANCE 

■WAVE METERS. RESONANCE IN SIMPLE CONDENSER CIRCUITS 

On account of the multiplicity of facts requiring presentation 
in an elementary discussion of electric wave phenomena, it is often 
difficult to decide what is the most direct course to follow. For 
a part of the way, in the earlier chapters, we were able to proceed 
almost in the historic order. Up to about the year 1900, the 
growth of knowledge of electric waves, so far as pertains to wireless 
telegraphy, occurred as a fairly direct sequence of important 
events, which have been sketched in Chapters I to XIII. About 
the year 1900 the literature of the subject began to multiply 
enormously and practical progress began to develop in many 
directions. Two main branches of this development we have 
already pursued, in a discussion of the propagation of the electric 
waves to long distances over the surface of the earth and in a 
discussion of some of the detectors used in receiving the signals. 
We shall now begin the study of a third main branch of the sub- 
ject; namely. Electrical Resonance. 

Introduction to a Study of Electrical Resonance. — In previous 
chapters attention has been called to the importance of bringing 
different parts of the sending and receiving circuits into resonance 
with one another. By this means tin 1 strength of the signals is 
increased, and the interference arising when several stations are 
operated simultaneously is partially' eliminated. 

The main elements of variation in attuning circuits one to 
another are inductance and capacity'. Preparatory' to the study 
of more. complex cases of resonance, let us recall the experiments 
of Sir Oliver Lodge, described in Chapter VIII, in which two Ley- 
den-jar circuits were attuned to each other. One of the Ley'den- 
jar circuits, which I shall call the oscillating circuit, was provided 
with a spark gap, and was charged by an electric machine and 
allowed to discharge. The other Ley den-jar circuit (compare Fig. 
142) was at a distance of perhaps a meter or two from the oscil- 
lating circuit, and could be adjusted as to period of vibration by a 

215 



ELECTRICAL RESONANCE 


217 


tory circuit, and by adjusting the slider S of Circuit II this circuit 
may be brought into resonance with the Circuit I of unknown 
period. The condition of resonance is indicated by the maximum 
glow in a sensitive vacuum tube in contact with one of the plates 
of the condenser of the frequency meter. When this resonant 




Fig.^ 43. Drude’s resonant method of measuring wave-length 
and frequency, 

adjustment has been made, the position of the pointer P on the 
scale is read, and from this reading the period of the frequency 
meter is known, for by calculation Drude has calibrated the fre- 
quency meter in terms of the period corresponding to any par- 
ticular adjustment of the pointer on the scale. 

The period of the frequency meter at resonance is the same as 
that of the oscillating circuit; which is, therefore, also known. 

Likewise, the wave length in air that Circuit I emits is known, 
for this wave length is the velocity of light times the* period. 1 

In terms of units, 

Wave length in motors = 3 X 10 s X period in seconds. 

By means of this apparatus Druclo was able to determine wave 
lengths between 2 and 445 meters. 

Doenitz’s Wave Meter. — Dr. Johann Doenitz 2 oi Berlin, Ger- 
many, has constructed a wave meter that is in a very compact and 
convenient form for measuring the wave lengths of wireless teleg- 
raphy. Instead of a gradually variable inductance, as in Drude s 
apparatus, Doenitz’s instrument has a gradually variable condenser 

1 See Chapter X. 

2 Elektrotechnische Zeitschrift , Vol. 24, pp. 920-925, 1903. German Patent, 
No. 149,350, from April 4, 1903. U. S. Patent, No. 763,164, filed Sept. 15, 
1903, issued June 21, 1904. 



ELECTRICAL RESONANCE 


219 


approaches resonance with the oscillating circuit whose wave 
length is to be measured. Resonance is determined by noting 
the amount of current in the wave-meter circuit. This is done 
by means of a Harris or Riess hot-wire air thermometer h 7 which 
is, however, not connected directly into the wave meter circuit, 
but is coupled with it by means of the oscillation transformer ii x . 
The action of this transformer and thermometer is as follows: 
The primary i of the transformer is in the wave-meter circuit; the 
secondary ii of the transformer is in series with a resistance vl, 
designed to be heated by the current through it. This heating of 
the resistance heats a quantity of air in a glass bulb surrounding 
the resistance, causing this air to expand, and to push up a column 
of mercury in the bent tube h . As the wave meter approaches 
resonance with the oscillation circuit, the rise of the column of 
mercury in the bent tube increases. 

By reading this indicator, not only can one determine the reso- 
nant adjustment of the wave-meter circuit, but one can also form 
some idea of the sharpness of resonance by noting whether small 
or large variations of the condenser are required for a given rise 
of the indicator. 

The range of wave lengths measurable by Doenitz’s wave meter 
is changed by substituting various coils of different numbers of 
turns for the receiving loop .s*. For each of the coils there 1 is a 
corresponding calibration of the scale 4 . 

Sample of Observations Made with a Doenitz Wave Meter. — 
The curves of Fig. 14/> were obtained! 1 by a Foenitz wave meter. 
The curves show the scale 4 rending of the 4 air thermometer for vari- 
ous settings of the wave meter. Curve I v as obtained by tuning 
the wave meter to an oscillating antenna circuit; Curve II was 
obtained by tuning the 4 wave* meter to an oscillating condenser 
circuit. The condenser circuit and the antenna circuit arc seen 
to have the same wave length, 320 motors, indicated by the fact 
that this value, 320 meters, is the reading of the wave-meter scale 
when the thermometer scale reading is a maximum. *Now r when 
the two circuits of curves I and II were coupled together, and the 
wave meter applied to a study of the oscillations occurring in the 
coupled system, the results plotted in Curve III were obtained. 
The resonance curve in this case has two maxima. To this subject 
we shall return. 

1 Figure 145 is copied with some slight modifications from Lieutenant- 
Commander S. S. Robison’s Manual of Wireless Telegraphy, 1906. 



ELECTRICAL RESONANCE 


221 


An advantage of Fleming's cymometer over other forms of 
wave meter arises in the fact that the scale readings are nearly 
proportional to the wave length (giving a nearly uniform scale 
when calibrated in wave lengths), whereas with instruments of 
the Doenitz type the wave length is nearly proportional to the 
square root 1 of the capacity of the adjustable condenser, so that 
the divisions on the scale become wider apart as the wave length 
increases. 

Fleming's instrument has, however, the disadvantage of lack 
of compactness, for the inductance and condenser of this instru- 
ment are from one to two meters long. 

Pierce Wave Meter. — I have designed a wave meter that has 
met with some use in practical application to wireless telegraphy. 
It consists of a Korda semicircular plate condenser C (Fig. 147), in 
series with a loop L for receiving the inductive action, and in 



Fig. 146. Fleming cymometer. 

series with a specially constructed high-frequency telephone 
receiver T. A pointer carried by the axle of the movable plates 
of the condenser passes over a scale, which is calibrated directly 
in wave lengths. 

At resonance, a maximum sound is produced in the high-fre- 
quency telephone receiver. On account of the high sensitiveness 
of the telephone receiver the wave length of currents in which 
the oscillations are extremely feeble may be determined, and also, 
on account of this high sensitiveness, the condenser can be made 
very compact and light, so that the whole instrument in the 
standard form weighs only 14 pounds. _ 

1 This the reader may verify by examining the formula X = LC. 



ELECTRICAL RESONANCE 


223 


an accurate device, called a “ stroboscope,” for determining the 
period of revolution of the mirror. 

Having the period of revolution of the mirror .and the distance 
between spark-terminal images on photographs like those of Fig. 3, 
one has a direct measurement of the period T of the discharge of 
a given oscillating circuit. By constructing a large number of such 
oscillatory discharge circuits giving various periods of discharge, 
or, better, by using a discharge circuit whose period could be varied 
at will, one may obtain accurate values of various periods by the 
use of the revolving mirror; and from the various periods T one 
can obtain the wave length X in air of the emitted wave by the 


formula 


X = v X T, 


where » = 3X 10 s meters per second, T is the time in seconds of 
one complete oscillation of the circuit, and X is wave length in 
meters. 

The wave meter to be calibrated is now set to resonance with 
each of these known wave lengths and the wave length is written 
at its appropriate position on the scale of the instrument. 

Another method of calibrating a wave meter is by tuning it 
to resonance with circuits of which the period is known by calcu- 
lation from a knowledge of capacity and inductance. 

Method of Using a Wave Meter. — Let it he required to deter- 
mine the wave length in air emitted by the oscillation circuit S, 



Fig. 148. The wave meter must be placed in such a position that 
the magnetic force from S links with the loop L of the wave meter; 
the oscillations in S then act inductively on the wave meter. 
This action is a maximum when the loop L is close up to S and in 
a plane parallel with it. It is, however, not advisable to have the 
two circuits too close together, because in this case the oscillations 



ELECTRICAL RESONANCE 


225 


The last column contains eight independent determinations of 
the capacity with an average error of only 1%. 

This is one of the best methods of determining the capacity of a 
condenser under conditions of actual use. 

Effect of Resistance on. the Sharpness of Resonance. — In tun- 
ing a condenser circuit with adjustable capacity or inductance to 
resonance with an oscillating circuit, as was done in the wave- 
metrical experiments above described, we have a simple case of 
the kind of tuning that is made use of at a receiving station when 
it is desired to receive signals of one wave length and exclude signals 
of a different wave length. 

One of the main difficulties in completely excluding undesired 
signals' arises from the fact that the detectors used in receiving 
the signals have a high resistance. 

Let us see how the sharpness of resonance is affected by resist- 
ance of the receiving circuit, in the simple case in which a con- 
denser circuit (e.g., the wave-meter circuit) .is attuned to a given 
wave length. 

As an example, I shall take a case in which the constants of the 
receiving circuit are within the range employed in wireless teleg- 
raphy. In Fig. 149 suppose that L is an inductance of .0001 
henry, / an instrument for measuring the 
oscillatory current (root of mean square cur- 
rent) produced by an incoming electric 
wave, which is supposed to have a wave 
length \i = 300 meters; C is a variable ca- 
pacity, and this capacity is supposed to be 
calibrated directly in wave lengths, X 2 . Let 
the receiving circuit be set at various wave 
lengths and let the corresponding current 

be read on the instrument /. Fig. 149. Simple oscil- 

i t , x x - , i lation circuit. 

By a calculation that is not here repro- 
duced, it can be shown that the results plotted in Fig. 150 will be 
obtained. The relative current is plotted vertically, while the 
settings of the wave length of the receiving circuit divided by the 
wave length of the incident wave (X 2 /Xi) are plotted horizontally. 

The different curves in the diagram show the effects of putting 
different values of the resistance R into the receiving circuit. A 
maximum current is received in each case when X 2 = Xi, but the 
sharpness or flatness of the curves depends on the value of R . When 
R = 628 ohms the top curve is obtained. This curve is nearly 




ELECTRICAL RESONANCE 


227 


la tms problem I have supposed that the waves which are 
moving are themselves undamped. If they also have strong 
fomping . the interference would be a little greater than that 
described, but the main imperfections of tuning are due to the 
resistance of the receiving station and not to the lack of purity of 
the wave from the sending station. The illustration shows that 
we cannot get very sharp resonance so long as we have to use a 
high resistance (the detectors) in the particular receiving circuit 
here employed. This difficulty is, however, considerably reduced 
by the use of coupled circuits at the sending and receiving stations, 
in the place of the simple condenser circuit of this computation. 

In the next chapter some facts in regard to resonance with 
coupled circuits will be presented. 



RESONANCE — OSCILLATIONS OF COUPLED SYSTEMS 229 


Simplified Form of Circuits. — In order to simplify the con- 
ditions somewhat, in the present experiments, instead of employ- 
ing the wireless telegraph circuits with the antenna constituting 



Fig. 151. Inductively coupled Fio. 152. Direct coupled transmit- 

transmitting station, ting station. 



Fig. 153. Inductively coupled condenser Fig. 154. Direct coupled con- 
circuits, with the antenna and ground denser circuits, 

of Fig. 151 replaced by a condenser. 


the capacity of the secondary circuit (such an antenna being in 
the form of a capacity distributed along a wire also possessing 
inductance), this antenna, for the purposes of these experiments, 
is replaced by a condenser, so as to have a localized capacity in 






RESONANCE — OSCILLATIONS OF COUPLED SYSTEMS 231 

In. Fig- 155, which represents the inductively connected system, 
two condensers Ci and C 2 are connected to two coils L 3 and i 2j 
which are inductively related but insulated from each other. The 
number of active turns of wire on each of the coils may be varied; 
X, 2 is varied by the clip contacts, and is varied by a wheel 
contact that may be moved along the inner spiral by a rotation 
of the drum on which the inner spiral is wound. 

Each of the condenser circuits is provided with a spark gap, so 
that either circuit, when connected to a step-up transformer, may 
be used as the discharge circuit. The other circuit may then be 
looked upon as a secondary circuit. . When the spark gap of the 
secondary is opened too wide to permit the passage of a spark, 
or, what is the same thing, when the secondary is removed, the 
period of oscillation is the period of the primary alone. When, 
on the other hand, the secondary is left in place and the spark gap 
of the secondary is closed (compare Fig. 153), the oscillations of 
the discharge circuit C\ Li induce oscillations in the secondary 
circuit C 2 L 2 , and we have a periodic flow of current in both 
circuits. It is proposed to give an account of some measurements 
of the wave length produced in the circuits when uncoupled 
and then when coupled with each other, and to compare the 
measured values with values computed from certain useful 
formulas. 

In the Direct Coupled System , represented in Fig. 156, which was 
also studied, the transformer of the inductive coupling is replaced 
by an auto-transformer; that is, the two condensers Ci and Co are 
made to discharge through parts of the same coil. In this case, 
also, both the inductances L\ and Lo can be varied independently 
by the motion of the contacts W and S. Also, both the condenser 
circuits are provided with spark gaps, so that either circuit may be 
caused to oscillate alone or to constitute the discharge circuit in a 
connected system with closed secondary. 

These two forms of circuits, Figs. 155 and 156, are derived from 
the ordinary wireless telegraph circuits by replacing the antenna 
and ground of the wireless telegraph station by the two coatings 
of a condenser respectively. The circuits in these simplified 
forms will yield results that will aid in understanding the actual 
wireless telegraph circuits, which are to be examined in subsequent 
chapters. 

Dimensions of the Inductances. — The coils employed in the 
apparatus shown in Figs. 155 and 156 had the following dimensions: 



RESONANCE — OSCILLATIONS OF COUPLED SYSTEMS 233 

convenient objects; for example, the backs of two chairs: The 
pendulum bobs may be any two small bodies of about the same 
weight — two heavy nails will do. At first make the lengths of the 
threads supporting the two pendulum bobs the same. Now leave 
one of the bobs at rest, pull back the other in a direction at right 
angles to the plane of the strings, and then release it. Note what 
happens. Try the effect of making the cross cord tighter or looser, 
and also the effect of making the two pendulums of unequal length. 

The vibratory motion of the pendulums represents very well 
the electrical vibratory motion that takes place with the coupled 
condenser circuits. 

Oscillograms of the Pendulum Motion. — In order to show 
graphically the nature of the pendulum motion, I have elaborated 
the pendulum apparatus a little, and taken a moving picture 



Fig. 158. Coupled pendulum with iirrangr-mnit for photo- 
graphing tlio motion. 


(oscillogram) of the motion of each of the pendulum bobs. To do 
this, a camera was placed in the position, shown at C in Fig. 158. 
At the back of the camera is a small horizontal slit A, and back of 
this slit is a sheet of bromide paper F carried by a rotating drum I). 
In order to have a bright object upon which to make the exposure, 
a small Nernst glower (7 was hung just above one 4 of the pendulum 
bobs. This Nernst filament was put into an electric circuit bv 
means of the small wire W> which also served as the suspension 
for the pendulum, and by moans of the return wire R , which was 
carried up in such a maimer as not to interfere with the freedom 
of motion of the pendulum. The current was started in the glower 
by heating it with a match while the current was on. As the 
pendulum swung, the image of the Nernst glower moved back and 
forth along the slit A. A small horizontally moving point of light 
thus entered the slit and fell upon the film. If now the sensitive 




RESONANCE — OSCILLATIONS OF COUPLED SYSTEMS 235 


n 

show the displacement of the bob plotted vertically, against time 
plotted horizontally. 

The first curve P, of Fig. 159, was obtained by leaving the 
ball M initially at rest, and pulling aside and releasing ball L 
(Fig. 158). The motion here corresponds to the primary current 
in the coupled condenser circuits. The second curve S was 
obtained by leaving the ball L initially at rest and releasing M . 
This curve corresponds to the secondary current of the coupled 
condenser circuit. The two cords supporting L and M were of 
the same length in the case of these two experiments. 

As another experiment, the two cords were both equally short- 
ened, and the transverse supporting cord was loosened; the curves 
P' and S' were obtained for the motion of the ball L initially dis- 
placed (primary) and initially at rest (secondary) respectively. 

The curves P and S or P' and S' represent very well the electrical 
vibratory motion of the coupled condenser circuits, if we think of 
the displacement of the bob in the two curves as representing 
the current in the primary and secondary circuits of the coupled- 
condenser oscillation. 

How the Curves Show the Existence of Two Periods. — Each 
of the curves of Fig. 159 shows the existence of two periods, in the 
motion of the pendulum, by the presence of “beats.” If two vibra- 
tions of different periods coexist in the same system, the slower of 
these vibrations will fall more and more behind the other in phase 
■until the two vibrations become just opposite to each other and 
neutralize each other; then the slower vibration will again fall 
more and more behind till it is a whole vibration behind the faster, 
and the two vibrations will then add and intensify each other. 
This is what has happened in the experiment with the pendulums. 
The same thing happens with the electrical vibrations of the con- 
denser circuits that are coupled together. 

Theoretical Values of Wave Lengths in the Coupled Circuits. — 
Let us now return to the experiments with the condenser circuits. 
By the use of the wave meter we can pick out and measure each 
of the periods or the corresponding wave lengths of the connected 
system of condenser circuits. When this has been done, we shall 
find that the wave lengths obtained satisfy the following theoretical 
relations 1 : 

1 Lord Rayleigh, Theory of Sound; J. v. Geitler, Sitz. d. k. Akad. d. Wiss. z. 
Wien, February and October, 1905; B. Galitzine, Petersb. Ber., May and June, 
1895; V. Bjerknes, Ann. der Physik, Vol. 55, p. 120, 1895; Oberbeck, Ann. der 



RESONANCE — OSCILLATIONS OF COUPLED SYSTEMS 237 


TABLE XHI 

INDUCTIVELY CONNECTED SYSTEM 

Primary capacity .00432 microfarad. 

Primary inductance varied . 

Secondaiy capacity .00482. 

Secondary inductance 24 turns outer coil, L. 2 = 6.60 X 10 - 6 henry. 
Wave length of secondary X 3 = 1060 meters. 


Turns Primary. 

Li 

Primary Inductance. 
Henry. 

M 

Henry. 

r 2 

60 

15.85 X 10~ 5 

6.52 X lO" 5 

.412 

45 

13.9 

6.14 

.421 

40 

11.8 

5.80 

.430 

35 

10.0 

5.12 

* .397 

30 

8.20 

4.45 

.360 

25 

6.50 

3.56 

.295 

20 

4.82 

2.70 

.228 

15 

3.15 

1.95 

.183 

10 

1.72 

1.20 

.128 

5 

.69 

.47 

.048 

3 

.32 

.23 

.0277 


Turns 

Primary. 

Meters. 

Calculated. 

Observed. 

V 

Meters. 

V 

Meters. 

Meters. 

V 

Meters. 

50 

1560 

1740 

727 

1750 

710 

45 

1460 

1670 

712 

1650 

685 

40 

1350 

1567 

686 

1570 

465 

35 

1230 

1462 

680 

1480 

660 

30 

1130 

1390 

660 

1370 

660 

25 

1000 

1273 

685 

1280 

660 

20 

870 

1185 

680 

1185 

630 

15 

700 

1127 

595 

1125 

565 

10 

510 

1080 

467 

1090 

460 

5 

300 

1060 

292 

1040 

285 

3 

210 

1062 

193 

1075 

210 


The method of taking; the observations is as follows: First, the 
condenser C 2 ( = .00482 mf.) was connected in series with 24 turns 
of the outer coil (Fig. 155) and was provided with a spark gap. 
In this position, with the inner coil thrown out of circuit by dis- 
connecting both plates of its condenser, the wave length X 2 was 
found to be 1060 meters. Next, with the secondary condenser 
disconnected, the wave length of the primary (inner) circuit was 
determined with its condenser C i ( = .00432 mf.) connected in 
series with 50 turns of the inner coil. This wave length Ai was 
1560 meters. Next, with the primary left unaltered, the second- 
ary was closed by attaching its condensers without spark gap to 
the 24 turns, of the outer coil. This is the case of the closed second- 




RESONANCE — OSCILLATIONS OF COUPLED SYSTEMS 239 


obtained both by measurement and by calculation. The observed 
and calculated results are plotted in the curves of Fig. 161. In 
t.hia case also the agreement is fairly satisfactory. 

These two experiments with the inductively connected system 
of circuits give an experimental verification of the formulas 
(1), (2), (3), (4) and (5), and serve to show how the wave lengths 
obtained with the connected system depend on the constants of 



200 400 600 800 1000 120( 

Xi Meters 4 4+4 Observed 

r. n o o calculated 


Fig. 161. Curves of wave lengths obtained with inductively coupled 
condenser circuits having individually the same period. 


the two circuits. Wo shall return to this subject after giving 
briefly the results of an experiment with the direct coupled system 
of circuits. 

Experiment with the Direct Coupled Circuit. — Co = .00178 
Microfarads, L 2 — 25.5 Turns = 6.7 X 10 Henrys, X 2 = 645 
Meters. — The apparatus for this experiment with the direct 
circuit is shown in Fig. 156. The steps of the experiment are 
similar to those with the other system of circuits. The observed 
and calculated values of the wave lengths in the compound oscil- 
lating system are plotted in Fig. 162. The formulas of calculation 
are the formulas (1) and (2), and the agreement between the 
observed and calculated results (crosses and circles) is seen to be 
satisfactory. 


RESONANCE — OSCILLATIONS OF COUPLED SYSTEMS 241 


When r is equal to unity the coupling is said to be perfect *md the 
equations (1) and (2) become 

X/ = vV + X 2 2 ; 
and X 2 ' = 0. 

That is to say, the oscillation, as shown also by method I, 
becomes single- valued. 

The case of perfect coupling was not observed in the experiments 
with the inductively coupled system, because for perfect coupling 
the primary and secondary coils must have the same number of 
windings and the two coils must be so close together as to be 
practically coincident, — conditions that could not be realized with 
the inductive coupling. 

Close Coupling and Loose Coupling. — One of the most interest- 
ing facts derivable from an examination of the equations (1) and 
(2), which are verified by the experiments, is the influence of the 
coefficient of coupling (r) on the wave lengths produced by the 
coupled circuits. In general, two wave lengths are obtained when 
a coupled system of circuits is set into oscillation. This duplicity 
of the wave leiffeth is often an inconvenience in wireless telegraphy, 
because, to avoid interference when a neighbor is sending a mes- 
sage we do not wish to hear, it is necessary to tune to avoid, not 
one undesired wave, but two. 

The influence of the coefficient of coupling on the wave length 
is very easy to investigate in case the primary and secondary of 
the coupled system are attuned to the same wave length X, as they 
generally are in practice. In this ease, the formulas for the com- 
pound wave lengths X/ and X 2 ' become the simple forms of equation 
(4) and (5); namely, 

(XxO 2 = X 2 (1 + r), (4) 

and (X 2 ') 2 = X 2 (1 — r). (5) 

Dividing each of these equations by X 2 , and extracting the square 
root, we have, 


x/ / 

X N 1+7 

(<) 

~=Vl-r 

A 

(8) 


Now, putting in various values of r = (.1, .2, .3, etc., up to 1.0), 
we obtain the relative values of X/ and X/, shown in the curves of 
Fig. 164. 



CHAPTER XXII 


TUNING THE SENDING STATION 

Having investigated, in the preceding chapter, the condi- 
tions of resonance and the manner of vibration of two condenser 
circuits connected together, it is proposed now to consider the 
actual wireless telegraph sending circuits. For this purpose let 

us examine the method of 
adjusting the direct coupled 
or the inductively coupled 
sending station to resonance. 
A diagram of a direct coupled 
sending station is shown in 
Fig. 165. The condenser C, 
repeatedly and periodically 
charged from a transformer 
Tr, discharges through a spark 
gap G and a few turns P of 
a “ helix.” The oscillations in 
this circuit act inductively and 
produce oscillations in the an- 
tenna circuit consisting of the 
antenna, the coils S of the 
helix, and the ground E. A 
maximum effect is produced 
when these tw'O circuits are 
properly adjusted to each 
other. A photograph, Fig. 
Fig. 165. Direct coupled transmitting is g j ven to show the con- 

struction of the sending helix 
(right) and a method of inclosing the spark gap for reducing the 
noise of the spark. 

A diagram of the inductively coupled sending circuit is shown 
in Fig. 167. Here the primary and secondary inductances are 
parts P and S of two separate helices. These two helices may be 
one above the other, as represented in the diagram, or may be one 

243 






TUNING THE SENDING STATION 


245 


meter WM up near the helix. The lower end of this helix is con- 
nected through a spark gap to the ground. The secondary of the 
station’s transformer is connected about the spark gap. The 



Fig. 168. Showing construction of the helices of an inductively coupled 

transmitting station. 


antenna is connected by means of a clip contact K to some particu- 
lar number of turns of the helix. The transformer is set into opera- 
tion so as to produce a spark at the gap. 1 This sets up oscillations 
in the antenna circuit, and the wave meter is adjusted to resonance 
with these oscillations. The wave length is read, and this reading 

1 In this case, where the spark gap is in the antenna circuit, there is a tend- 
ency for the spark to go over into an arc and not produce good oscillations. 
This may be obviated by playing a small blast of air on the spark. 



TUNING THE SENDING STATION 


m 



Fig. 170. Curves showing wave lengths of antenna circuit and 
condenser circuit with different numbers of turns of the helix. 



Fig. 171. Method of measuring wave length of the condenser circuit. 





TUNING THE SENDING STATION 249 

Adjustment of Direct Coupled Sending Station to Resonance 
with' the Aid of a Hot-wire Ammeter. — Another method of 
adjusting the condenser circuit and the antenna circuit to resonance 
makes use of a hot-wire ammeter, inserted in the antenna circuit as 
represented at A, Fig. 165. This instrument contains a fine wire 
through which the oscillations pass, producing heat. The heated 
wire expands, and by means of a delicate gearing attachment, the 
sagging of the expanding wire acts upon a hand passing over a 
dial. The movement of the hand over the dial is thus an indica- 
tion of the amount of current passing through the sensitive wire. 
The instrument may be calibrated directly in amperes, but this 
calibration (chiefly on account of the shunts that have to be 
employed) is without much absolute value, when the hot-wire 
ammeter is used with the very rapid oscillations of wireless teleg- 
raphy. Nevertheless, a maximum deflection of the instrument 
indicates a maximum of current in the antenna, and this is all 
that is required of the hot-wire ammeter in order to decide when 
the antenna and condenser circuits are in resonance. 

Instead of inserting the hot-wire ammeter in the antenna above 
the helix, it may just as well be placed in the lead from the helix 
to the ground. In either case oscillations in the antenna circuit 
pass through the instrument. 

To tune up a station with a hot-wire ammeter, let the station be 
coupled up as shown in Fig. 165. Bet the transformer in action, 
and read the hot-wire ammeter. Now keeping the spark gap con- 
stant, and leaving the antenna clip K unchanged, move the clip 
K' of the condenser circuit to a different number of turns of the 
helix, and again read the current. Make a table containing the 
number of turns of helix in primary circuit and corresponding hot- 
wire ammeter readings. Then plot a curve of readings against 
turns in the form shown in Fig. 173. From this figure it is seen 
that the maximum reading of the ammeter was obtained when the 
primary was discharging through 1.3 turns of the helix. This is, 
therefore, the adjustment that must be given to the primary in- 
ductance in order to bring the condenser circuit into resonance with 
the antenna circuit, for the fixed value of the secondary induc- 
tance employed throughout the adjustment. 

Since the readings of the hot-wire ammeter depend on the values 
of the mean square current through it, one can, by a process like 
that described, find out just what conditions of the two circuits 
give the greatest mean square current in the antenna, and if 



TUNING THE SENDING STATION 


251 


the primary and secondary helices brings the two resultant wave 
lengths produced by the station closer together, and gives a sharper 
wave system than that obtained with a large co effi cient of cou- 
pling. The coefficient of coupling of the direct coupled system 
also may be varied, for example, by introducing more or less 
inductance (not mutual) in one of the circuits. 

The question as to the best coefficient of coupling to employ 
at the transmitting station is difficult to decide. The question 
is complicated by the conditions that exist at the receiving station 
as well as at the sending station. I shall therefore defer a con- 
sideration of this question until after a discussion of the resonant 
relations at the receiving station. 

The Detuning of Coupled Circuits. — We have shown in the 
preceding paragraphs how the condenser circuit and the antenna 
circuit may be adjusted to resonance. This gives in the coupled 
system a maximum flow of current and a maximum radiation of 
energy from the antenna. The energy radiated is, however, in 
the form of two waves of different wave lengths. Suppose this 
doubly periodic wave to be received by a receiving circuit. Can we 
not tune the receiving circuit either to the one or to the other of 
the received wave lengths? And would it not be preferable to 
adjust the transmitting condenser circuit to a little longer or a 
little shorter wave than the transmitting antenna circuit in order 
to strengthen the longer or the shorter wave of the coupled system 
at the expense of the other wave which is not to be used at the 
receiving circuit? Professor M. Wien 1 shows that a small ad- 
vantage (in some eases as great as 30%) may be derived from a 
process of this kind provided the condenser circuit and the antenna 
circuit are differently damped. In his experiments Wien used 
a simple, low-resistance receiving circuit, and I am unable to say 
how great would be the advantage in a similar detuning operation, 
when the coupled receiving circuits and the high-resistance de- 
tectors of actual practice are used at the receiving apparatus. 
In my own experiments I have never detected any appreciable 
advantage in detuning an actual sending station. 

Possible Existence of Three Wave Lengths in a Coupled Sys- 
tem. — With the condenser circuit and the antenna circuit attuned 
to the same independent wave length, as in the case of our wave 
metrical illustration on page 248, there is the possibility of the 


1 Annalen der Physik, Vol. 25, p. 1, 1908. 



CHAPTER XXIII 


SOME RECENT METHODS OF EXCITING ELECTRIC WAVES 
THE SINGING ARC, THE SINGING SPARK, AND THE 
QUENCHED SPARK 

Thus far in this account, practically only one method of pro- 
ducing oscillations at the sending station has been described; 
'namely, the method making use of the spark discharge of a con- 
denser which has been charged from an alternating current 
transformer or an induction coil. Electric waves produced in 
this way occur in discrete trains. 

Recently several new methods of exciting the oscillations have 
come into use. We shall begin the discussion of these newer 
methods by describing the “ singing arc,” which is a wide departure 
from the ordinary spark discharge. The singing arc operates on a 
direct current source, produces a practically continuous sequence 
of waves, and has met with application, not only to wireless teleg- 
raphy, but also to wireless telephony. The history of the sing- 
ing arc may be traced back more or less connectedly to an early 
experiment by Elihu Thomson. 

Elihu Thomson’s Continuous Current Spark. — In 1892 Pro- 
fessor Elihu Thomson 1 found that electric oscillations could be 
produced from a 500-volt direct current source by connecting the 
source through a resistance with a spark gap which was shunted 
by a condenser and inductance. This form of circuit is repre- 
sented in Fig. 174. A source of direct electromotive force of 500 
volts is shown at E. This is connected in series with a resistance 
R and a spark gap. In parallel with the gap a condenser C and 
a self-inductance L are shunted. Under these conditions electric 
oscillations were found to be present in the condenser circuit. In 
the effort to intensify and steady the effects Professor Thomson 
used a blast of air or a magnet to blow out the spark. This appa- 
ratus of Professor Thomson with some modifications and improve- 
ments has been reverted to in some of the recent developments of 
wireless telegraphy and telephony. 

1 U. S. Patent, No. 500,630, July 4, 1892. 

253 



RECENT METHODS OF EXCITING ELECTRIC WAVES 255 


an inductance With proper adjustments of the various parts 
of the circuit the arc emits a musical sound which in Duddell’s 
experiments could be plainly heard to a distance of several meters. 
The pitch of the note can be varied by varying the capacity C or 
the inductance S. The experiment is highly interesting when one 
varies the capacity C by means of a set of keys and thereby pro- 
duces a succession of notes of different pitches. 

In addition to the evidence afforded by the emission of musical 
sounds, the shunt circuit comprising the condenser C, the induc- 
tance L and the arc A, may be shown also by its inductive action 
on a neighboring circuit to be traversed by a pulsating or oscillat- 
ing current. We have thus a pulsating or oscillating current pro- 
duced from a direct-current source. 

Why the Arc Gives Rise to Pulsating Currents. — The expla- 
nation of the production of oscillatory currents and audible sounds 
by the arc shunted with a condenser has been the subject of a 

considerable amount of theo- 
retical and experimental inves- 
tigation. 1 Duddell’s original 
account of the phenomenon con- 
tains a simple explanation, which 
is substantially as follows : 

The electric arc between car- 
bon terminals has a falling volt- 
ampere characteristic like that 
shown in Fig. 177. With an in- 
crease of current through the 
arc the voltage between the are 
terminals decreases. For this 
reason, when the arc is connected 
in series with a source of voltage 
and is “struck” by bringing the 
terminals together and then sep- 
arating them, the current through the arc tends to increase to a 
very large value, and must be restrained by a suitable resistance 
R in circuit with the arc (see Fig. 176). 

Suppose, now, that when the arc is quietly burning, a condenser 
C and inductance S are together connected about the arc. The 

1 For a theoretical treatment of this subject the mathematical reader is 
referred to an article by H. Th. Simon, Physikalische Zeitschrift, Vol. 7, p, 433, 
1906. 



Fig. 177. Volt-ampere character- 
istic of carbon arc. 



RECENT METHODS OF EXCITING ELECTRIC WAVES 257 


primarily in placing the arc in an atmosphere of coal gas or hydro- 
gen , and in employing for the arc one terminal of carbon ( — ) and 
the other terminal of a water-cooled cylinder of copper (+) (cf. 
Fig. 178). For the purpose of effecting the cooling of the copper 
electrode, it was made hollow, and through it a stream of water was 
circulated. Water was also circulated through a worm within 
the jacket inclosing the coal gas or hydrogen about’ the arc, so as 
to prevent undue heating of this jacket. To enhance the strength 



and the frequency of the oscillations, the poles of a powerful electro- 
magnet NS are inserted, gas-tight, into the chamber, and placed so 
as to give a magnetic field transverse to the arc. The carbon 
terminal of the arc is slowly rotated by a clockwork or electric 
motor. This is to prevent the formation by the arc of inequalities 
in the surface of the carbon electrode 4 . When all of these pre- 
cautions indicated by Poulsen are taken, the oscillations may be 
given a frequency as high as a million or more per second, 1 which 
brings them well within the range useful for wireless telegraphy and 
telephony. 

The source of current is a direct current generator D , giving 

1 By the use of an arc having a water-cooled copper cathode and a silver- 
point anode, N. Stschodro (Ann. d. Phys. Vol. 27, p. 225, 1908), has obtained 
more than 300,000,000 oscillations per second, and has performed Hertz’s 
mirror experiment with the electric waves so produced. 




recent methods of exciting ELECTRIC WAVES 25d 


frequency can be obtained, even with the arc in air. The sur- 
rounding of the arc with an atmosphere of hydrogen permits these 
high-frequency oscillations to be obtained also with a large current 
(10 to 12 amperes) through the arc, which is a valuable asset for 
the sustenance of energetic oscillations. 

Instead of employing an atmosphere of hydrogen about the arc, 
ordinary coal gas, such as is used in illumination, produces also 
very good results. 

One method of feeding the gas into the chamber is to lead it in 
continuously by a rubber tube connected with the gas jet of the 
illuminating system. The gas, after passing through the chamber 
about the arc, is conducted away by a rubber tube leading to the 
outside of the building, or else it is led to a gas burner and 
ignited to prevent it from escaping unconsumed into the room. 

The Use of Other Hydrocarbon Gases and the Use of Steam 
About the Arc. — Instead of coal gas or hydrogen, almost any 
other gaseous hydrocarbon may also be employed with the arc to 
enhance the energy and improve the con- 
stancy of the high-frequency oscillations. 

For example, the combustion products of 
an alcohol flame will produce effects in a 
degree similar to effects with the coal gas. 

These combustion products may be sup- 
plied to the arc by means of a small alcohol 
lamp placed beneath the arc, as is shown 
in Fig. 180. 

Similar beneficial effects upon the oscillations are produced by 
the gases formed by the volatilization of a liquid hydrocarbon, 
such as turpentine, pentane, amyl alcohol, etc. In this case the 
liquid hydrocarbon is allowed to fall drop by drop into a cup- 
shaped depression in one electrode, where it is volatilized and 
surrounds the arc with an atmosphere of gas. 

Dr. Lee DeForest 1 has suggested steam as an atmosphere for 
the arc, and has shown several methods of supplying steam to the 
arc. One of these methods is depicted in Fig. 181 taken from 
DeForest's United States patent specifications. 

Use of Several Arcs in Series. — To obviate the necessity of the 
magnetic field and the coal-gas atmosphere, as used with the 
Poulsen arc, the Telefunken Company of Germany employs several 
arcs in series, thus obtaining a high effective voltage. Only a 
i TJ. S. Patent, No. 850,917, issued April 23, 1907. 







RECENT METHODS OF EXCITING ELECTRIC WAVES 263 


Instead of having the interrupter or detuning vibrator at the 
sending station, it may be used at the receiving station, as has 
been proposed by Poulsen. 1 A diagram of a circuit in which this is 
done, taken from Mr. Poulsen's United States patent specification, 
is shown in Fig. 183. The receiving antenna circuit a is induc- 
tively connected with the condenser circuit &, c , d. In shunt about 
the condenser d is a detector s with its accessories. A vibrating in- 
terrupter at / is adapted to connect another condenser k periodically 
in parallel with the condenser c. 

When the contact is interrup- 
ted at /, assuming that the os- 
cillation circuit is tuned to 
resonance under these circum- 
stances, intense oscillations 
will appear in this circuit, and 
by means of the detector at 
$, which rectifies the oscilla- 
tions, an integral current will 
pass through the telephone. 

If now the contact at / is 
closed, the circuit is thrown 
out of resonance, oscillations 
in the circuit 6, c, d cease, 
and the current in the tele- 
phone ceases. When the con- Fl °* 183 * deceiving circuit for persist- 
f . . , ent waves (Poulsen). 

tact at / is again opened, 

another integral current passes through the telephone, which in 
this way is made to respond with a sound of pitch determined by 
the frequency of the interrupter. 2 

In order to obviate the necessity of these interrupter devices 
at the sending or receiving circuit, an Einthoven galvanometer 
at the receiving station may be used instead of the receiving tele- 
phone. Einthoven's instrument will respond to the uninterrupted 
train of waves and possesses a sensitiveness even greater than the 
telephone receiver. The deflections of this galvanometer may be 
photographically recorded. Although this instrument, with the 
necessary moving film for taking the photographic record of the 
message, is not quite so simple to install or to operate as the circuit 

1 U. S. Patent, No, 897, 779, applied for March 6, 1907, issued Sept. 1, 1908. 

2 The explanation given by Mr. Poulsen in his patent specification is incon- 
sistent with the explanation here given. 




RECENT METHODS OF EXCITING ELECTRIC WAVES 265 


that the interference difficulties, even, with undamped waves, are 
still considerable. 

The main advantage in the singing-arc method of excitation 
arises in the applicability to wireless telephony of this method of 
producing electric oscillations. Wireless telephony is briefly con- 
sidered in a subsequent chapter. 

As a continuation of the discussion of novel methods of pro- 
ducing oscillations, we shall next describe the Lepel arc and the 
quenched spark. 

The Lepel Arc. — In a German Patent, No. 24,757, filed Aug. 20, 
1907, Baron von Lepel has described a very simple and efficient 
form of discharge gap which is capable of operating on either a 
direct or an alternating-current source. It consists simply of 
two circular discs of copper with a thin sheet of paper between 
them. The discharge occurs between the discs and through the 
paper. A small perforation made near the center of the paper 
affords a suitable starting place for the discharge. As the dis- 
charge continues, the paper is gradually burned away from the 
center outwards. This burning away takes place in an atmos- 
phere deficient in oxygen, and consequently requires several hours 
to use up all the paper. A circular groove cut near the outside 
edge of the adjacent faces of the copper plates prevents the arc 
from getting to the outer edge of the discs and there being exposed 
to the air. The essential feature of the Lepel gap is that the spark 
or arc shall be very short and shall occur in the space which is 
deficient in oxygen. The presence of the products of combustion 
of the paper enhances the efficiency of the arc. The arc will 
operate on a direct current source, and gives discrete trains of 
oscillations of which the pitch may be made very high and may 
be regulated by regulating the condenser about the gap and the 
rheostat placed in the leads to the current supply. 

The series of discharges obtained from the direct-current supply 
occurs in a manner resembling the occurrence of the series of dis- 
charges obtained by Elihu Thomson with his singing spark, as 
described on page 253. In addition each discharge is rapidly 
quenched and gives the quenched-spark effect described under 
the next headingc 

The discs of the Lepel arc are 3 to 5 inches in diameter, and, for 
rapid conduction away of the heat generated, are made of copper 
or silver, which have high conductivity for heat. The discs may 
also be made hollow, and are then cooled by the admission of 



RECENT METHODS OF EXCITING ELECTRIC WAVES 267 

corresponding electrical case. P and S represent the current in 
the primary and secondary oscillating circuit having in the primary 
an ordinary spark gap. P' and S' represent the current in the 
primary and secondary of a system having a quenched spark in 
the primary. The spark is quenched when the energy in the 
primary attains its first minimum. If this spark does not recover 
its conductivity again, the secondary oscillation continues with 
its own free period and damping as represented in S'. 

Now it has been shown that a very short spark kept. well cooled 
has exactly this characteristic of rapidly ext inguishing after a 


Primary and Secondary, Ordinary Spark 



Fig. 184. Curves showing oscillations with ordinary spark and with quenched 

spark. 

few oscillations, as is represented by the curve P\ A method of 
attaining a similar result with a comparatively large amount of 
power consists in using several gaps of the Lepel type in series. 

This has been done by the Telef unken Company in Germany 
with marked success. A diagram of the quenched spark, com- 
prised of several minute gaps in series between metal discs, is shown 
in Fig. 185. The face of one of these discs, which are of copper, 
is shown in the upper part of the figure. The lower part of the 
figure shows a section of a pile of these discs, placed so as to give 
several of the gaps in series. Between each pair of the discs is a 


recent methods of exciting electric WAVES 269 


surfaces. One gap will carry efficiently not more than 4 amperes. 
The oscillations occur in a practically continuous train and are 
suitable for wireless telephony. To give a tone to the discharge, 
so as to adapt it to wireless telegraphy with a rectifier and tele- 
phone as receiver, one of the discs may be segmented. 

Some Facts in Regard to the Quenched Spark. — Recurring to 
the curves of Fig. 184, it will be seen wherein consists the advantage 
of a properly quenched spark; namely, the spark is active only long 
enough to allow the oscillations of the antenna circuit to build 
up to a maximum of intensity. The number of oscillations of the 
primary requisite to attain this is the fewer the closer the coupling 
between primary and secondary. The intensity of the secondary 
is a maximum when the current of the primary is a minimum. If 
the spark completely loses its conductivity at this point, the sub- 
sequent oscillations of the secondary induce an electromotive force 
in the primary, but if no current is established in the primary, no 
energy is thereby consumed, and all of the energy, which is now 
stored in the secondary circuit, will stay there until radiated. 

If, on the other hand, the primary spark does not completely 
lose its conductivity at its minimum, the e.m.f. impressed back on 
the primary by the oscillations in the secondary will reestablish 
current in the primary. This current in the primary, flowing as it 
does repeatedly across the spark gap, heats it, and dissipates a 
considerable part of the energy of the system as heat in the gap. 
This recommunication of energy to the primary is worse than use- 
less because in addition to dissipating energy, it is active also in 
burning away the spark gap and in severely straining and heating 
the transmitting condensers. 

In addition to this loss of energy and the destructive strain on 
the apparatus, the double periodicity of the vibration, with the 
use of the unquenched spark, is a hindrance to discriminating 
tuning of the receiving station. 

The quenched spark is, therefore, economical in transmitting 
energy, and is favorable to sharp tuning; and, by obviating a use- 
less dissipation of energy in the primary circuit, it also materially 
contributes to the life of the transmitting apparatus. 

What are the characteristics of a spark gap in order that it 
should give a quenched spark ? After the energy has left the 
primary circuit, the gap should very rapidly recover its high resist- 
ance, so that oscillations will not again be set up in the primary 
by the reaction of the secondary. This the author found to be 



CHAPTER XXIV 


RESONANCE OF RECEIVING CIRCUITS. THE POSSIBILITY OF 
PREVENTING INTERFERENCE 

How does the current induced in a receiving antenna depend 
upon the height of the receiving antenna? How much is the 
strength of this current modified by tuning the antenna ? In a 
coupled receiving circuit what resonant relations exist between 
the two parts of the coupled system ? How sharp is the tuning 
at the receiving station, and to what extent can interference be 
prevented ? 

It is proposed in this chapter to present a brief examination of 
these questions. 1 For this purpose .some experiments are described. 

DEPENDENCE OP RECEIVED CURRENT ON HEIGHT OF RECEIVING 

ANTENNA 

In an investigation to ascertain the dependence of received 
current on the height of receiving antenna, a direct coupled 
transmitter, like that illustrated in Figs. 152 and 165 was used to 
produce the electric waves. The two circuits of the transmit- 
ting station were adjusted to resonance with each other by the 
hot-wire ammeter method of Chapter XXII. The dimensions 
of the transmitting circuits were as follows: The secondary part 
S of the helix consisted of 5 turns of wire .208 cm. in diameter, 
wound in a spiral 40 cm. in diameter, with a pitch of 5.08 cm. The 
inductance of this part of the helix was 1.56 X 10 -5 henrys. The 
primary part P of the helix consisted of 1.2 turns and had an 
inductance of .151 X 1CT 5 henrys. The condenser was made up 
of sheets of copper separated by miconite plates. The antenna, 
with dimensions marked, is shown in Fig. 186. The station sent 
out two waves, — one of wave length 153 meters and the other 
of wave length 129 meters. 

For the purpose of determining what relative currents are 

1 G. W. Pierce: Physical Review, Vol. 19, p. 196, 1904; Vol. 20, p. 220, 
1905; Vol. 21, p. 367, 1905; Vol. 22, p. 159, 1906. 

271 



RESONANCE OF RECEIVING CIRCUITS 


273 


The variable inductance used for tuning the circuit consisted of 
51 turns of wire, .208 cm. in diameter, wound in a spiral on a 
vulcanite drum. Variations of inductance were made by turning 
the drum, and thereby causing a wheel-contact to move along the 
spiral. The inductance of the whole coil was 16.5 X 10 ~ 5 henrys, 
and the inductance of 
any fraction of the coil 
was accurately known. 

The results of a set of 
measurements are given ie 
in the curves of Fig. 188. 

The first curve, marked a 
23.2 at its vertex, was | 
taken with a vertical re- | 10 
ceiving antenna 23.2 me- Q 
ters long (measured from 
the junction with the B 
tuning coil). The differ- 
ent points on this curve 
were obtained as deflec- 
tions of the dynamome- _ _ 

ter for different values of Inductance x 10 Henry 

the inductance of the p I( , iss. Resonance curves with circuit of 
tuning coil. When the form of Fig. 187. 



length of the receiving 

antenna was changed from 23.2 meters to 20 meters, the curve 
marked 20 tit, its vertex was obtained. In the same way the 
curves marked 16, 12 and 8 were obtained 
for lengths of antenna 10, 12 anti 8 meters 
respectively. 

Before discussing the results of this experi- 
i["j Q[ | t lent I will present data obtained with a 

U — 1 different form of receiving circuit. 

[~kl Similar Experiments with Shunt-Capacity 

Fig. 189. Circuit Method of Tuning. — A diagram of this 
for tuning with receiving circuit is shown in Fig. 189. An 

Shunt capacity. a( jj us table air condenser of known calibration 

in terms of capacity was placed in shunt to the receiving instru- 
ment, I, and by its use tuning was effected. Different lengths 
of receiving antenna were employed and the resonance curves 
of deflections against capacity were plotted. These are given 



RESONANCE OF RECEIVING CIRCUITS 


275 


obtained with the series-inductance method. The deflection at 
resonance for the two different methods of tuning, for different 
heights of antenna, are plotted in Fig. 192. The lower curve A 
was obtained with the inductance method of tuning; the curve B, 



2 4 8 8 10 12 14 16 18 20 22 24 

Height o t Antenna, Meters 

Fig* 192. Deflection as a function of the height of antenna. A, circuit 
tuned with series inductance; B, tuned with shunt capacity. 

with the shunt-capacity method. It Is seen that the shunt- 
capacity method of tuning gives larger values. In comparing 
these results numerically it should be remembered that the deflec- 
tions of the instrument are proportional to the square of the 
current received. 

Relation of Received Current to Height of Receiving Ante nna — 

Coming now to the more important question as to the relation of 
received current to height of receiving antenna for each of the 
methods of tuning, we get the interesting result that the law is 
entirely different for the two different methods. 

In order to make the relation apparent, the scale of the deflec- 
tions was changed by a constant multiplier so as to make the 
deflection at 23.2 meters unity. The simplified relative deflec- 
tions thus obtained, together with the square roots and the fourth 
roots of these deflections are plotted in Figs. 193 and 194. It is 
seen that in the series-inductance case (Fig. 193) the square- 
roots of the deflections lie on a straight line, while in the shunt- 
capacity case (Fig. 194) it is the fourth roots of the deflections 
that lie on a straight line. 

Remembering that the deflections of the instrument are pro- 



RESONANCE OP RECEIVING CIRCUITS 277 

as they should for an exact proportion. The reason of this de- 
parture from proportionality in the case of Law II may be found 
in the fact that the lengths of antenna were measured from the 
instrument to the top of the antenna. This leaves out of account 
the part of the antenna between the instrument and the ground, 
which amounted to 2 meters. This was also exposed to the action 
of the waves, and should perhaps be added to the height; this 
would make Law II almost an exact statement of the experimental 
result. 

It is entirely possible that the relations I and II here stated may 
fail of verification when tested with greater heights of antenna. 
In the meanwhile the relations may be taken as fair approxima- 
tions to the truth. 



Fig. 1 95. Transmitting and receiving circuits for resonance experiments. 
RESONANCE IN INDUCTIVELY COUPLED RECEIVING CIRCUIT 

In the present experiments the inductively coupled type of 
circuits was employed at both the sending and the receiving sta- 
tions. These circuits are shown in Fig. 195. It is seen that the 





RESONANCE OF RECEIVING CIRCUITS 


279 


n&ting electric light circuit. The secondary of the transformer 
was connected to the condenser C, Fig. 195. The switch in the 
primary was closed and opened automatically by a clockwork, 
so that the signals were sent every 35 seconds, without the aid 
of an assistant. Each signal lasted for 5 seconds, which was a 
little greater than the time required for reading the receiving 
instrument. 

Receiving Instrument. — The receiving instrument, shown at 
G, Fig. 195, was again the high-frequency dynamometer (described 
on p. 113), with a resistance of 1.33 ohms. Such an instrument 
of low resistance does not materially modify the resonance condi- 
tions, so that the results obtained are the results for the circuits 
themselves. When these circuits are employed with the com- 
mercial detectors of high resistance, it is necessary to ascertain 
how far the resonance relations are modified by the detector. 
At present, however, we are concerned primarily with the resonant 
behavior of the circuits themselves. 

Harmonic Oscillation. — The following experiment shows the 
possibility of harmonic resonance of the inductively coupled 



10 20 30 40 60 60 70 80 90 100 110 120 130 140 150 160 

Receiving Capacity 

Fig. 196. Resonance curves obtained by taking readings of the dynamometer 
with various adjustments of the sending and receiving condensers. 


sending and receiving circuits. With the sending and receiving 
antennae circuits of identical dimensions, different values were 
given to the capacity of the sending station, and resonance curves 
were taken by variations of the receiving capacity. The curves 
of Fig. 196 were obtained. Curves 1, 2, 3, ... 7 were with 
1, 2, 3>j , , ,7 plates of condenser at the sending station. It is 


RESONANCE OF RECEIVING CIRCUITS 


281 


change in the antenna circuit. The reason is apparent. The 
15 plates set the antenna vibrating with its fundamental period, 
while the 3 plates set the antenna vibrating as a first odd harmonic. 
The plates of the sending condenser were not all equal, so we must 
look to the receiving apparatus for a verification of this statement. 
This verification is evident from the optimum values of the reso- 
nant receiving capacity; namely, approximately 108 and 12, which 
are in the ratio of 9 to 1. These capacities being in the ratio of 
9 to 1, the corresponding periods, which are proportional to the 
square root of the capacities, are in the ratio of 3 to 1, which is the 
ratio of fundamental to first odd harmonic. 

This evidence of the possibility of a harmonic excitation of the 
sending antenna, and the harmonic response of the receiving 
antenna, shows the interesting analogy of the electrical apparatus 
to such acoustic apparatus as a closed organ pipe. 

This experiment was performed with the receiving antenna cir- 
cuit an exact duplicate of the sending antenna. For the purpose 
of obtaining information somewhat more general, it is proposed 
next to show some experiments with variations of the length of the 
receiving antenna, and to study the resulting effects on resonance. 

Resonance Curves with Variation of the Length of Receiving 
Antenna. — The inductively coupled transmitting station S of Fig. 
195 was employed to produce the waves. The sending antenna 
used was the four-wire antenna 15.8 meters long of Fig. 186. The 
sending condenser circuit was carefully adjusted to resonance with 
the antenna. The conditions at the sending station were kept 
constant. 

At the receiving station, which was also inductively coupled 
(cf. R, Fig. 195), the coils of the inductive coupling were kept 
constant. The problem was to set up at the receiving station 
various heights of antenna, make various adjustments of the con- 
denser in the side circuit and take readings of deflections of the 
dynamometer which is in the side circuit. 

We have arriving at the receiving station waves of constant 
period and approximately constant intensity, and we are to seek 
the conditions under which the receiving instrument shows the 
largest readings. The variables are the height of the receiving 
antenna and the capacity of the air condenser, which is in the side 
circuit at the receiving station. 

The receiving antenna of four wires was started at a height of 
23.8 meters, measured from the coil in the mast circuit. The 



RESONANCE OF RECEIVING CIRCUITS 283 

plotted horizontally and the height of the antenna plotted ver- 
tically. Curve A was found by trial to have approximately th e 
equation 

(H a - 11.8) (C 4 . - 84.6) = 88, (a) 



10 20 60 40 BO 60 70 80 90 100 110 120 130 140 ISO 160 
Receiving Capacity 


Fig. 199. Relation of resonant receiving capacity to height of 

receiving antenna. 


as is shown by the following comparison of observed values, with 
values calculated from this equation (Table XIY): 


TABLE XIV 

RELATION BETWEEN HEKiHT OF RECEIVING ANTENNA AND RESONANT 


CAPACITY. FOUR WIRES RECEIVING 


Curve No., 


Maximum 

Resonant 

Resonant 

Meters Antenna 

Capacity Ob- 

Capacity Cal- 

Fig. 106. 

Above Coil, ll a . 

Deflection, cm. 

served, C 4 

ciliated. 

1 

23.8 

64 

92 

91.9 

2 

20.8 

47 

94 

94.4 

3 

17.8 

43 

100 

99.3 

4 

15.8 

29.5 

106 

106.6 

5 

14.8 

21 

115 

113.9 

6 

13.8 

13 

130 

128.6 

7 

12.8 

7.5 

165 

172.6 


The only large difference between the observed and the calcu- 
lated value of resonant capacity is in the case of Curve 7, where 






RESONANCE OF RECEIVING CIRCUITS 


285 


The two groups when plotted with resonant receiving capacity 
against height of antenna form a curve of two branches A, A', 
Fig. 199. Values calculated from the equation (a) are plotted as 
the dotted lines in Fig. 199. The heavy curves are the observed 
values. From a comparison of the observed values with the com- 
puted values, we see that our equation, although it led us to look 
in the right direction for the resonance, is yet an imperfect equa- 
tion. There are other terms in it beyond those here set down. 



Fid. 200. Various types of inductively coupled receiving circuits. 


Applicability of these Experimental Results to Practice. — One 
may ask, what is the use of this experiment in which the receiving 
transformer is kept constant and the length of antenna and the 





RESONANCE OF RECEIVING CIRCUITS 


287 


shows that the following is approximately 1 the relation between 
the several wave lengths in order to produce a maximum current 
in the condenser circuit: 


/! _ 1 ..)/L_l\ == z 2 

V XV Vx a 2 xv X 4 ’ 


( 1 ) 


in which r is the coefficient of coupling at the receiving station. 

By a maximum current in the condenser circuit one or another 
of the maxima of the twelve different curves of Fig. 198 is meant. 
Not all of these maxima are equally strong, nor is the resonance 
for all of the maxima equally sharp. But for nearly any value 
of X 0 we can get a value of \ c that will give resonance of a more 
or less pronounced character. 

Let us try a few numerical examples that will make this clear. 
Let r = .20 ; and suppose waves are arriving of wave length X = 400 
meters. Suppose that our antenna wave length is set at X tt = 300 
meters. Then we have 

r = .20, 

X = 400, 

X u = 300, 

to determine X c . With these numerical values equation (1) 
becomes 

$ L _ i l $ i !_ ( = (Q.20)* 

Ik* (400) 2 $ ((300) 2 (400) 2 ) (400) 4 ' 


Multiplying by (400) 4 we get 


/ 400 a 

\Xc 2 



') 


.04. 


Whence A c = 390 meters. This 390 meters is the wave length 
at which we must set our receiving condenser (in a coupled circuit) 
in order to receive a 400-meter wave, provided our antenna is set 
for a 300-meter wave. 

Carrying through similar computations for other values of the 
wave length of the incident waves we obtain the results recorded 
in Table XV. 


1 In the derivation of this formula the small effect of resistance on the 
wave length was neglected; also the capacity of the antenna was considered 
localized instead of distributed. The formula (of which our equation (a) is 
a special case) is, therefore, inexact, but will serve to illustrate some interest- 
ing facts about the tuning of a receiving station. 



RESONANCE OF RECEIVING CIRCUITS 


289 


This curve shows several facts of interest. It shows, for example, 
that when we have been receivings wave length slightly shorter 
than our antenna wave length, and a wave comes in slightly longer 
than our antenna wave, we must actually decrease our receiving 
capacity to bring the longer wave into resonance. It shows also 
that any particular adjustment of our receiving capacity is reso- 
nant for two different waves. For example, with our antenna set 
at wave length 300 meters, and our condenser circuit set for 400 
meters, we are really in tune for either a 290-meter wave or a 410- 
meter wave, not in the best tune, it is true, but sufficiently in 
tune to be disturbed if the interfering signals are strong. 

Advantage of Varying Coefficient of Coupling in Tuning. — 
There are times when we wish to be in tune for two wave lengths 
at once, because the station we are receiving usually sends out 
two waves at once. If we set our receiving condenser at 300 
meters, we are in tune for a 270-meter and a 330-meter wave, 
and these might well be sent out by the same station. They will 
in fact be sent out by the same station, if it has the same coefficient 
of coupling as our receiving station, r = .20, and has its condenser 
circuit and antenna circuit tuned to 300 meters. 

This suggests an important improvement in our tuning mech- 
anism at the receiving station; namely, a device by which we can 
change the coefficient of coupling at the receiving station and thus 
make the receiving coefficient of coupling identical with the coeffi- 
cient of coupling of any particular station we wish to receive. 
This device 1 is employed in many of the recent receiving sets, 
and consists of an adjustment by which the primary coil of the 
receiving transformer may be either moved away from or rotated 
with respect to the secondary coil. The same result can be at- 
tained by cutting out inductance in the primary of the transformer 
and putting it in series where it will not be in inductive relation 
with the secondary coil. 

Effect of Variation of the Coefficient of Coupling on Sharpness 
of Resonance and on Received Energy. — Theory shows that 
diminution of the coefficient of coupling increases the sharpness 
of resonance. At the same time this diminution of coefficient of 
coupling brings with it a decrease of energy. I tried some experi- 
ments to see what improvement in sharpness of resonance we might 

1 On. account of the high resistance of the detectors the proper adjustment 
of the coefficient of coupling is not one of exact equality with the coefficient 
of coupling of the sending station, but must be determined by trial. 



RESONANCE OP RECEIVING CIRCUITS 


291 


effect of resistance of detector on resonance in coupled 
WIRELESS TELEGRAPH CIRCUITS 

Although the coefficient of coupling of the coupled circuits 
influences somewhat the sharpness of resonance, a far greater 
influence in the case of the practical stations is 
exercised by the resistance of the detectors 

which are used in the reception of the signals. 

These detectors, when sufficiently sensitive to 

respond to weak signals, have a very high re- c s 

sistance. We have seen in Fig. 150 (p. 226) 

how a high resistance inserted in a simple circuit r - 

consisting o'f a condenser in series with an in- c K L 

ductance renders the resonance dull. With the i Jr fe 

coupled circuits the effects are somewhat more tjJ <3 £r, 
difficult to present, and it is necessary to examine /g) J S 

the resonance curves obtained by varying both k* . 

the antenna wave length and the condenser- ✓ Hj 

circuit wave length in order to ascertain the | 

influence of resistance on the sharpness of e 


resonance. 


Fio. 204. Diagram 
I have submitted the problem to a mathe- of circuit proyid- 

matical examination, and without giving the of waveTJngtlio'f 
steps of the reasoning, I take the liberty of pre- primary and see- 
senting some of the' results. The form of re- able^eondensers? 
ceiving circuits to which the discussion applies 
is shown in Fig. 204. The following constants of the circuits 
were assumed in the computations: 

1 / 3 = Self-inductance of the antenna circuit = .3 X 10 ~ 3 henry, 

Li — Self-inductance of the condenser circuit = .5X 10~ 3 henry, 
M = Mutual Inductance = .2 X 10~ 3 henry. 
r 2 = Square of coefficient of coupling = .267, 

X = wave length of incoming waves = 472 meters. 

The antenna circuit was given various resistances, R 3 , and the 
condenser circuit various resistances, R> . The resistance Ri 

resides chiefly in the detector, and the resistance R a includes the 
apparent resistance due to distributed capacity in the antenna.. 
The incoming waves were supposed to be a persistent train of 
undamped waves. 

Computations were made for two cases: I, When we fix the 
antenna adjustments at their best values, and tune with C 4 ; 





RESONANCE OF RECEIVING CIRCUITS 


293 


Case I (Continued). R^ = io Ohms, R4 = 10,000 Ohms. — 

Suppose, now, that the detector should have 10,000 ohms resistance 
instead of 64,000 ohms. With this reduced resistance the curve 
marked “ Ra = 10,000 ” is obtained. With this value of Ra, tun- 
ing by the condenser Ca is possible, but the resonance is dull as 
is indicated by the obtuseness of the curve. 

Appropriate adjustment of the antenna in this case is at the 
line marked “ 10,000 ” on the bottom margin; namely, X 3 = 470 
meters. 

Case I (Continued). R3 = 10 Ohms, R4 = 1000 Ohms. — The 
curve marked “ Ra = 1000 79 is obtained; and the antenna must 
be shifted to the line on the bottom margin marked “1000”; that 
is, the antenna wave length must be set at 460 meters for best 
resonance. The resonance curve “ Ra = 1000 ” is much sharper 
than those obtainable with the higher resistances. 

Case I (Continued). R« — 10 Ohms, R4 = 100 Ohms. — 
Reference is made to the curve marked “ Ra — 100,” and to the 
line at the bottom margin marked “ 100.” The resonance is 
sharper than with the higher resistances, and the appropriate 
adjustment of antenna wave length has shifted to X* = 430 meters. 

Case I (Concluded). R3 = 10 Ohms, R4 = 10 Ohms. — Two 
resonance positions appear in this case: one at 400 meters (wave 
length of the condenser circuit), with appropriate adjustment of 
antenna at 360 meters; and the other at 610 meters (condenser 
circuit), with antenna adjustment at 810 meters. The resonance 
here is extremely sharp, especially for the adjustment of condenser 
Ca in the neighborhood of 400 meters. 

Case IL Let us now suppose a detector circuit of resistance 
10,000 ohms, and let us set the condenser Ca of this detector 
circuit at its resonant value in the neighborhood of 135 meters 
(see the diagram for Case I), and then tune with the antenna 
circuit; for example, by varying the condenser C T 3 . The results 
are given in Fig. 206, the different curves corresponding to different 
values of R s in the antenna circuit. From these curves it will be 
seen that even with a high-resistance detector ( Ra = 10,000 ohms) 
the tuning in the antenna circuit is sharp, provided the antenna 
effective resistance is low (curve marked “ Rz = 10 ”). With 
increase of antenna resistance the resonance becomes less sharp. 

In practice with a system of coupled circuits like that under 
discussion and with the high-resistance detectors in use, it is 
difficult to realize sharper resonance than that shown in the curve 



RESONANCE OF RECEIVING CIRCUITS 


to Fig. 207 that if a receiving station is attuned for a 500-meier 
wave, it will receive also about 7% as much energy from a 400- 
meter or a 600-meter wave as it does from the 500-meter wave. 
From a station emitting a 300-meter or a 700-meter wave the 
disturbing energy will amount to about 2 % of the energy received 
from the 500-meter wave; while from a sending station emitting 
a 200-meter or a 800-meter wave the disturbing energy will be 
below 1%. These statements are on the assumption that all of 
the stations would give the same received energy if the receiving 
station were in tune for them. 

These computations, although not claiming to be highly accu- 
rate, will give a crude idea of about the extent to which inter- 
ference can be prevented by. the use of the coupled circuits 
consisting of a condenser circuit containing the receiving instru- 
ment inductively or directly coupled to an antenna circuit. 

There are other methods of coupling receiving circuits- to pre- 
vent interference which will attain better discrimination between 
desired and undesired signals, but these almost always greatly 
reduce the intensity of signals, and cannot be employed for the 
reception of signals from stations at a great distance from the 
receiving station. 



DIRECTED WIRELESS TELEGRAPHY 


297 


bolic cylindrical surface and were connected to a spark terminal Si. 
Another similar set of strips B u B % , B s . . . below the first set 
were also provided with a spark terminal jS 2 . The oscillations are 
produced by a discharge across the spark gap S 1 S 2 . This arrange- 
ment, which, according to the inventor, would send out electric 
waves in one direction, does not seem to have met with practical 
success. 

Braun’s Phase-difference Oscillator. — Another method pro- 
posed by Ferdinand Braun 1 makes use of two or more vertical 
oscillators at certain distances apart provided with means of 




oscillator. 


Fio. 209. Braun's phase-difference oscilla- 
tor for directed wireless telegraphy. 


exciting in the oscillators waves suitably differing in phase. For 
example, if the two antennae A and B, Fig. 209, are one half wave 
length apart, and if the oscillations in the two antennae are 
opposite in phase, the two sets of waves sent out will add in 
directions in the plane of the two antennae and will neutralize 
each other in a direction at right angles to this plane. 

Suitable phase difference in the antennae may bo partially 
attained by the use of a condenser circuit coupled with the an- 
tennae, as shown in Fig. 209. With this arrangement the problem 
is, however, complicated by the occurrence of oscillations of 
double periodicity. This difficulty has been removed in a very 


1 U. S. Patent, No. 776,380, filed July 26, 1904, issued Nov. 29, 1904. 




DIRECTED WIRELESS TELEGRAPHY 


209 


Explanation of Directive Radiation from Marconi’s Bent 
Antenna. — Professor Fleming, 1 Dr. Uller, 2 Dr. Zenneck, 3 and 
others, have given explanations of the cause of the directive radia- 
tion from the Marconi horizontal antenna. All of these writers 
employ the theory of images as a starting point, by which means 
the antenna and ground connection of Fig. 210 is replaceable by 
the equivalent system of Fig. 212. 

Fleming’s Explanation. — In further explanation, Professor 
Fleming takes a rectangular circuit of the form shown in Fig. 213, 
and imagines a current flowing around the rectangle in the direc- 


A B 



Fig. 212. Marconi directed antenna and its image. 



Fig. 213. Diagram used by Professor Fleming in explanation o£ 
the directive action of the Marconi bent antenna. 

tion of the arrows. This current creates a magnetic field, the 
direction of which along the surface of the earth is at right angles 
to the plane of the paper; and at equal distances from the center, 
the magnetic force represented by H is toward the spectator on 
both sides. Now, suppose a wire EF equal in length to one side 
of the rectangle to be placed contiguous to one vertical side, and 
to carry a current opposite in direction to that in the side of the 
rectangle (left hand) to which it is in proximity; then the magnetic 
field of this straight current is h! from the spectator on the left- 
hand and h toward the spectator on the right-hand side. Accord- 
ingly, the total field H + h on the right is greater than the total 
field H — h' on the left, because, according to Professor Fleming, 
the individual fields are added on one side and subtracted on the 
other. Now, since the two oppositely directed currents in the 

1 J. Fleming: Phil. Mag., Vol. 12, p. 588-604, 1906, 

3 Carl Uller: Phys. Zeitsch., Vol. 8, p. 193, 1907. 

3 J. Zenneck: Phys. Zeitsch., Vol. 9, p. 553, 1908. 



DIRECTED WIRELESS TELEGRAPHY 


301 


Chapter XV, the electric force at the surface of the earth, where - 
ever it is not a good conductor, leans forward, so that we can 
ascribe to the electric force in a particular case a mean direction, 



Fig. 214. Dr. Uller’s diagram of field of electric force about the 

bent antenna. 


E , Fig. 215. Now the direction of propagation is perpendicular 
to E ; i.e., in the direction S, whence there is penetration of the 
energy into the earth's surface and a consequent absorption, so 



Fig. 215. Diagram used by Dr. Zen- Fig. 210. Zenneck’s diagram show- 
neck in explaining directed wireless ing the course of the radiation 
telegraphy. from A to R. 



Fig. 217. Diagram applying to Zenneck’s explanation. 

that the distant receiving station is reached by the energy that 
started in the direction AX, Fig. 216, and not by the energy that 
started along the surface of the earth. By examination of Fig. 



DIRECTED WIRELESS TELEGRAPHY 


303 


placed within the two coils m and n and is capable of rotation 
about an axis through o. 

Electric waves coming from any particular direction produce 
oscillation in the two antenna circuits with intensities respectively 
dependent on the direction from which the waves come. The 
oscillations thus set up, passing through the coils m and n, com- 
pound to form a single magnetic field with a direction perpendicular 
to that from which the waves come. The strength of the induced 
current in the movable coil s will depend on its orientation with 
respect to the resultant magnetic field, and will be a maximum 
when the coil s is in a position to embrace as many as possible of 
the lines of magnetic force. This optimum direction is perpen- 
dicular to the field, and therefore parallel to the direction from 
which the waves are coming. 

It is therefore possible to determine the direction from which 
the waves are arriving by merely providing the rotating coil s 
with a pointer in its own plane. When a maximum strength of 
signals is received the pointer is directed either toward or away 
from the signaling station. The final ambiguity as to whether 
the signaling station is in the direction of the pointer or in the 
opposite direction would have to be removed by some additional 
general knowledge of the probable location. 

A sending station, devised also by Bellini and Tosi, and capable 
of directively transmitting signals, consists of a similar aerial 
system and a similarly rotatable interior coil. The latter is, how- 
ever, connected with a discharge condenser instead of with the 
receiving mechanism. The processes involved are, then, the 
reverse of those entering into the receiving apparatus. 

Limitations of Directive Wireless Telegraphy. — The several 
directive devices above described act directively only in a general 
way; that is, some more energy is sent in one direction than in other 
directions, but there is still a considerable diffusion of energy in all 
directions. The economy effected in the energy of transmission 
does not seem to be very great, particularly because the closed 
loops, or nearly closed loops, are not such good radiators or receiv- 
ers as the straight vertical antenna. However, whenever the bent 
antenna is installed in land stations the orientation to effect maxi- 
mum transmission in the most useful direction is generally chosen. 
Also, it has been proved to be entirely possible with each of the 
principal systems to determine the direction of the receiving station 
from the sending station. This achievement does not seem to have 



CHAPTER XXVI 

WIRELESS TELEPHONY 

Sketch of the Method of Wireless Telephony by Electric Waves. 

The circuits employed in wireless telephony by electric waves 
resemble very closely those used in wireless telegraphy. 

The transmitting apparatus for wireless telephony mak es use 
of a persistent train of electric waves of high frequency sent out 
from an antenna. Instead of interrupting these electric waves 
by a key, as in telegraphy, modifications by the voice, correspond- 
ing to spoken words, are impressed upon them. These modifica- 
tions by the voice are applied to the electric waves by means of a 
carbon transmitter, or similar instrument, placed in the sending 
circuit or connected with it. 

The receiving apparatus is indentical with that employed in 
wireless telegraphy, and makes use of a receiving antenna coupled 
with a circuit containing some type of rectifying detector; e.g., an 
electrolytic detector, a crystal-contact detector, or a vacuum-tube 
rectifier. About the detector is shunted a sensitive telephone 
receiver. 

The action is as follows : If an unmodified train of electric waves 
having a frequency higher than the limit of human audibility 
(35,000 vibrations per second) arrives at the receiving station, the 
receiving circuit, if properly tuned, will sustain electric oscillations 
which, passing through the detector, will be rectified and will give 
a series of rectified impulses to the receiving telephone circuit. 
These impulses, being all in one direction, will act as a continuous 
pull on the telephone diaphragm, — a continuous pull for the 
reason that the diaphragm cannot follow the rapid successive 
impulses, and because also, on account of the inductance of the 
telephone circuit, these impulses are modified electrically into a 
practically continuous current through the receiver. 

Having in mind that a continuous train of high-frequency waves 
produces a continuous pull on the receiving telephone diaphragm, 
let us now suppose that words are spoken into a carbon transmitter 
at the sending station in such a manner as to modify the emitted 

305 



WIRELESS TELEPHONY 


307 


with it, a wave length suitable for wireless telephony, namely, 
3 X 10 8 /75,000 = 4000 meters. With this apparatus, Professor 
Fessenden reports that he has carried on telephonic communica- 
tion between Brant Rock, Massachusetts, using an antenna 440 
feet high, and New York, using an antenna 200 feet high. The 
distance between these two stations is about 200 miles. Recently 
Professor Fessenden also reports successful wireless telephonic 
communication between Brant Rock, Massachusetts, and Wash- 
ington, D. C., a distance of about 600 miles. 




Fig. 219. Professor Fessenden's a p- Fig. 220. Diagram of Vreeland’s 

paratus for wireless telephony, mercury-arc oscillator, 

using high-frequency generator I) 
and a microphone transmitter T. 


The Mercury-arc Method of Producing Sustained Oscillations. 
— In 1906 Mr. Frederick Vreeland 1 described a very interesting 
method of getting practically pure sinusoidal undamped oscilla- 
tions from a direct-current supply. One form of Mr. Vreeland's 
apparatus is shown in Fig. 220. T is a glass vessel, exhausted 
to a high vacuum, and containing a mercury cathode K and two 
carbon anodes A and J3. E is a small auxiliary electrode used in 
starting an arc in the chamber. The arc, when established, being 
fed from the direct-current source D, is divided into two branches 


1 Physical Review, Vol. 27, p. 2S6, 190S, 


WIRELESS TELEPHONY 


309 


up to the pitch required for wireless telephony. His apparatus is, 
however, very ingenious and full of promise. 

Method of Applying the Microphone to Modify the Oscilla- 
tions. — Having described methods of producing sustained or 



Fig. 221. View of Mr. Vreeland’s apparatus. 


persistent oscillations I wish next to show briefly diagrams of con- 
nections by which the carbon microphone may be applied to modify 
these oscillations in accordance with the vibrations of the voice. 
In most of these diagrams I have represented the source of the 
persistent oscillations as a singing arc, such as has been devised 
by Simon, Duddell, and Poulsen. It will easily be seen how these 





WIRELESS TELEPHONY 


311 


that the microphonia modifications of current have to traverse 
the generator circuit, and hence meet with high impedance. 

Figure 224 shows the microphone connected in series with the 
antenna circuit, between the secondary of the oscillation trans- 
former PS and the ground connection. 

Figure 225 shows a method proposed by Mr. Vreeland and 
others in which the microphone circuit is inductively connected 
with the secondary S of the oscillation transformer. 

Other methods of connecting the microphonic transmitter to 
the oscillating circuit are also employed. 

Practical Results in Wireless Telephony. — I have briefly pointed 
out in the preceding paragraphs the general processes employed 
in wireless telephony. The small amount of space here devoted 
to the subject is not to be taken as evidence that wireless telephony 
is a simple or unimportant branch of the science of electric-wave 
transmission of intelligence. 

To be able to modulate a train of electric waves by waves of 
sound existent in the air between the mouth of the speaker and a 
transmitting diaphragm, and to be able to receive these modulated 
electric waves at a distance and reconvert them into sound waves, 
is a very remarkable achievement of scientific ingenuity, even 
when the sending and receiving stations are close together. Wire- 
less telephony has, however, gone far beyond this stage; and 
Fessenden in America, Poulsen in Denmark, Majorano in I tab', 
and Messrs. Colin, Jeance and Mereier in France, have severally 
reported successful wireless telephonic transmission of speech to 
distances ranging from 40 to 600 miles. Even if these experi- 
ments have been lacking in some details of perfection, we cannot 
doubt that practical wireless telephony, especially between ships 
at sea at a considerable' distance apart, is a possibility of the 
present time or of the immediate future. 



CONSTRUCTION OF WIRELESS APPARATUS 


313 


travels out along the surface of the earth induces currents in the 
earth and is rapidly absorbed. The remainder of the energy 
radiated from this horizontal portion travels prevalently upward 
and, save for contributing to the directiveness of transmission 
as has been pointed out in Chapter XXV, does not have much 
effect at the receiving station unless it is desired to transmit 
to a balloon, when this upward-traveling component is most 
useful. 

The horizontal portion of the flat-topped antenna is, therefore, 
chiefly serviceable as a capacity at the top of the vertical part, 
which latter is the chief radiating member. As to the amount of 
the capacity it is interesting to note that a single wire 100 feet long 
and inch in diameter when alone in space has as much capacity 
as an isolated flat metallic disc 16 feet in diameter. (See formulas 
for calculation in Appendix II.) From this it will be seen that 
the horizontal top to the antenna is a far more economical elevated 
capacity than any kind of a metallic sheet such as was employed 
in Marconi’s early experiments. 

Comparison of Flat-topped with Straight Antenna. — In order 
to illustrate some of the principles involved, let us next compare 
the radiation from a single vertical wire 100 feet long and say J 
inch in diameter with that from a flat-topped antenna consisting 
of a vertical wire 100 feet long having at the top a horizontal 
extension of the same length. For the purpose of this comparison 
we shall employ the experimental curve of current distribution 
found in Chapter XIV (Fig. 82). In the first place the flat-topped 
antenna, because of its greater length of wire, has approximately 
twice as much capacity as the simple vertical antenna. This 
means that if we charge the two antennae to the same potential, 
about twice as much electricity will flow during one oscillation of 
the flat-topped antenna as during one oscillation of the simple 
vertical antenna; but the time of the oscillation in the former case 
will be about twice as long; therefore the maximum current flowing 
to the ground will be about the same in the two cases. Let us 
now plot the approximate current-distribution curves for the two 
cases,, assuming the same current at the base; and in doing this 
we shall make the further assumption that the distribution in the 
bent antenna is approximately the same as it would be for a 
straight antenna of the same length. The curves obtained are 
given in Fig. 227. In these curves the value of the current at any 
point of the length of the antenna is plotted as a distance between 



CONSTRUCTION .OF WIRELESS APPARATUS 


315 


base Is not multiplied in the ratio that the number of wires is 
multiplied. 

For an economical installation from four to six wires may well 
be employed in the antenna, and by the use of light bamboo 
spreaders they can easily be supported three feet or more apart. 

Marconi Antenna at Clifden. — An example of the use of the 
flat-topped antenna on a large scale is afforded by the Marconi 
high-power station at Clifden, Ireland. The horizontal part of 
the antenna of this station consists of 200 wires 1000 feet long 
supported 180 feet above the earth The wave length is about 
4000 meters. 

The Umbrella Antenna. — When only one supporting pole is 
available, either the straight type or the umbrella type of antenna 



Fkj. 228. Umbrella type of antenna. 


is usually employed. The umbrella type meets with frequent use 
in small amateur stations and in the portable stations employed 
by armies. In this type the aerial system consists of a vertical 
portion terminating above in a system of wires inclining downward. 
These inclining wires are usually the guy wires, while the vertical 
part may be either a wire leading to the top of the pole, or the 
pole may itself be of metal and serve as the vertical conductor. 
A diagram of an umbrella type of antenna with a metallic pole 
serving as the vertical conductor is shown in Fig. 228. The 




CONSTRUCTION OF WIRELESS APPARATUS 


31T 


ity of a wire the same length and f of an inch (1 cm.) in diameter. 
Therefore, so far as concerns capacity, a few small wires five or 
six feet apart would be the equivalent of this large steel tube. 

The Ground. — The theory of the action of the ground has 
been discussed in Chapter XIV. In practice, for a small station 
a satisfactory ground can be obtained by a connection to the pipes 
of a water supply. Where this is lacking, a good arrangement is 
to bury a netting or network of wires at a short depth below the 
surface of the earth. This may be supplemented by metallic pipes 
driven to considerable depths into the earth, and also by wire 
netting spread out on the surface of the earth. When the station 
is located near the sea or other body of water, the wire netting or 
wires provided with terminal plates may be led into the body of 
water. On board ship, the grounding is usually effected by a 
heavy wire attached to the metallic hull of the ship. In the high- 
power land stations, netting and wflres are made to ramify the 
surface of the earth for many acres. 

We have seen in Chapter XIV that a properly resonant artifi- 
cial conductor supported without contact with the earth serves 
as a very good ground. The difficulty about the artificial ground 
is the fact that the artificial ground should be tuned along with the 
aerial system in order to get resonance with different wave lengths. 

Sending Condensers for a Coupled Transmitting Station. — 
The details of construction of the simple Marconi apparatus of 
1896 need not be given. When a sending station of the inductivel\ r 
coupled or direct coupled type is to be employed, the sending con- 
densers must be electrically strong in order to permit the storage 
of the large quantities of electricity used in producing the waves. 
Among the types of condenser employed for this purpose the bank 
of Leyden jars or of flat glass plates provided with metallic coat- 
ings are most familiar. The use of tinfoil, for the coating of 
Leyden jars or flat-plate condensers for use in wireless telegraphy, 
has been largely discontinued. In the case of the flat-plate con- 
densers copper or brass sheets between the plates in the place of 
the tinfoil that was formerly much used gives a much smaller 
loss of energy, and consequently much smaller heating of the con- 
denser. Ordinary window glass, when selected free from flaws, is 
electrically stronger than plate glass for making glass-plate con- 
densers. When high power is to be used, the flat-plate condensers 
should be submerged in castor oil to prevent brush discharge. 

In the case of the Leyden jars, when used in stations of large 



CONSTRUCTION OF WIRELESS APPARATUS 


319 


voltage at which the discharge occurs. As a specific example, let 
us suppose that the power is to be supplied by an alternating 
current source of n cycles per second. By m eans of a transformer 
with its primary connected to the source of power and its secondary 
attached to the condenser, we may step up the potential to the 
value required to produce the required spark. Let us suppose the 
transformer to supply P kilowatts of power to the condenser, and 
let us choose the condenser and the spark gap to be such tha t 
the condenser charges to a sparking potential only once during 
each half-cycle ; that is, 2 n times per second. 

Now to charge a condenser once to a potential of V volts requires 
an amount of energy, 

W = i QV joules, (1) 

where Q is the number of coulombs of electricity required and 
| V is the average potential during the charge. (See Appendix I.) 

And, from the definition of capacity, 


Q = CV, 


( 2 ) 


where C is the capacity of the condenser in farads. 

Substituting the value of Q from equation (2) in equation (1), we 
have 

ir= \ CV- joules, Oi) 


V being the potential in volts to which the condenser is charged. 

In our supposed ease the condenser is charged 2 n times per 
second; therefore the energy expended per second, which is the 
power supplied, is 

W = 2 n X JCT 2 = nCY- joules per second. (4) 

But I joule per second is 1 watt, and 1000 watts make a kilowatt ; 
therefore if P is the power in kilowatts, 


P = 


nCT* 

1000 


kilowatts. 


(5) 


In interpreting this formula, it must be remembered that I’ is the 
potential to which the condenser is charged at the time that the 
spark begins. 

The formula (5) is very useful in practical computations. By 
a simple transposition of terms, equation (5) may be put in the 
form 

„ 1000 X Power in Kilowatts 



CONSTRUCTION OF WIRELESS APPARATUS 


321 


will draw a very small amount of power, and allow the spark to 
extinguish promptly after the discharge of the condenser. 

A mathematical examination of this problem shows that this 
result can be obtained with a proper adjustable resistance placed 
in the primary circuit of the transformer, if a common closed-core 
transformer is used. The same result can be more economically 
obtained by the use of an adjustable inductance in series with the 



primary. It can also he attained by an adjustable inductance in 
series with the secondary of the closed-core transformer. 

With an open-core type of transformer and ail adjust able induc- 
tance in the primary circuit considerably greater flexibility in 
attaining resonance with condensers of different capacities is pos- 
sible, and many engineers prefer the open-core transformer. 

Sending Helix. — The construction of the sending helices of 
the direct-coupled and the inductively coupled type is shown in 
the photographs of Figs. 166 and 168 respectively. 

Sending Key. — With power not exceeding 5 kilowatts at a 



CONSTRUCTION OF WIREL ES S APPARATUS 


323 


ground are joined to the primary of the inductively connected 
receiving transformer, and the line circuit is opened so as to avoid 
a possible accidental discharge of the high-potential circuit while 
receiving. 

A photograph of a station with approximately the arrangement 
of circuits here indicated is shown in Fig. 231. 

I will next describe some of the parts of the receiving appa- 
ratus, and shall employ in the description the designations used in 
Fig. 230. 

Receiving Condensers. — The series condenser, which is em- 
ployed in the antenna circuit between the primary of the receiving 
transformer and the ground, should be an air condenser of the 
semicircular plate type, like that shown in the photograph of 
Fig. 81. The introduction of this condenser has the effect of 
shortening the wave length of the antenna, so as to adapt an 
antenna of long wave length to receive short waves. Tuning by 


means of this condenser gives a better 
discrimination of signals according to 
their wave lengths than can be obtained 
by the use of adjustments in the detec- 
tor circuit; nevertheless this series con- 
denser can often be dispensed with. 

The secondary receiving condenser, in 
circuit with the detector, cannot be dis- 
pensed with. This condenser may also 
he of the semicircular air type, but its 
capacity should usually be larger than 
can be attained with a single condenser 
of this type. If the secondary of the 
receiving transformer is adjustable as 
to inductance, the secondary condenser 
does not require to be capable of fine 
adjustment, and a condenser with mica 
plates as dielectric, and provided with 
step-by-step adjustment, may be used. 
In fact, with adjustable inductances in 
the transformer, the value of the secon- 
dary condenser may well be entirely fixed. 



Receiving Transformer. — A photograph of one type of receiv- 


ing transformer is given in Fig. 232. The secondary coil of this 


transformer is shown near the top the apparatus. The primary 


CONSTRUCTION OP WIRELESS APPARATUS 


325 


voltage for the local circuit is taken from this resistance by two 
leads, one to the end of the resistance and the other to the sliding 
contact. The exterior of a potentiometer in which the resistance 



Fig. 233. View of a potentiometer. 


is wound on a circular collar and the sliding contact carried by a 
rotating arm is shown in Fig. 233. 

Two electrolytic detectors, mounted on a common base with 
this potentiometer, are shown in Fig. 234. 

With some of the crystal-contact detectors a small voltage in 



Fig. 234. Two electrolytic detectors with potentiometer. 


the local circuit may be an advantage. The potentiometer in this 
case need, however, employ only one dry cell or one Leelanehe cell. 

Reliance on Principles Rather than on Details. — The details 
of construction here given appertain primarily to what is at the 
present time the most, usual type of -wireless telegraph station. 
Progress in this respect is, however, very rapid, and it is not at all 



Abraham, M., theoretical value of 
wave length, 116. 

Absorption of electric waves. By 
soil, 127, 131; by ionized air, 137. 

Air. Absorption by, 137; Conduc- 
tivity of, 137. 

Air condenser, 318; of Korda, 114. 

Alternator, high-frequency, 306. 

Amesbury, Mass., experiments at, 
134. 

Analogy. Of self-inductance to in- 
ertia, 22, 28; of capacity to me- 
chanical quantities, 26, 28. 

Anatase, 177. 

Antenna. Dependence on height of, 
271; resonance with various lengths 
of, 281; theory of directive, 299; 
types of, 312, 313. 

Antenna circuit, determination of 
wave length of, 246. 

Apparatus, construction of, 312. 

Arc. Mercury, 307; singing, 253; 
talking, 254; pulsating, 255; in 
steam, 259; period of singing, 260. 

Armagnat, characteristic of electro- 
lytic detector, 203. 

Arons tube, 72. 

Atlantic cable, 63. 

Atomic structure of electricity, 8. 

Attenuation of electric waves, By ab- 
sorption, 127; by divergence, 129. 

Attraction, electrostatic, 329, 

Audibility, limit of, 148. 

Auction, 214. 

Austin, L. W. Sensitiveness of tele- 
phone receiver, 140; detector, 160, 
198; electrolytic detector a recti- 
fier, 203. 

Balloons, 89. 

Barretter, 154; liquid, 203. 


Bell, Graham, telegraphy by conduc- 
tion through water, 77. 

Bellini, directive wireless telegra- 
phy, 302. 

Bjerkness, waves on wires, 70. 

Blondlot, waves on wires, 68, 71, 72. 

Bolometer, 72, 153. 

Bornite, 134, 161. 

Bose, short waves, 60. 

Boys, C. V., radiomicrometer, 129. 

Brandes, H., characteristics of de- 
tectors, 171. 

Branly, E., coherer, 80, 143. 

Brant Rock, Mass., tower at, 316. 

Braun, Ferdinand. Coupled cir- 
cuits, 101; artificial ground, 121; 
cathode tube, 151, 181; directed 
wireless telegraphy, 296, 297. 

Break key, 90. 

British stations, 326. 

Brookite, 177, 187. 

Calibration of wave meter, 22, 117. 

Calzecchi-Onesti, coherer, 80. 

Capacity. Electrostatic, 22; of con- 
denser, 24; of earth, 24; analogy, 
26; measured by wave meter, 224; 
amount at sending station, 318; 
formulas for, 339, 340. 

Cape Race, 106, 107. 

Capillary electrometer, 142. 

Carbon microphone. As detector, 
158; applied to wireless teleph- 
ony, 309. 

Carbon-steel detector, 158, 198. 

Carborundum. Detector, 160; ex- 
periments with, 162; unilateral 
conductivity, 164; current-voltage 
curves of, 164, 165, 167, 169, 171; 
oscillograms of, 187. 

Cathode tube, 151, 181. 



INDEX 


345 


Dissipation, of charge by ionized air, 
138. 

Distance. Law of, 130; of trans- 
mission over different soils, 131. 

Doenitz, Johann, wave meter, 216. 

Dolbear, Amos, wireless telegraphy 
of, 77. 

Double oscillation, spark photograph 
of, 248. 

Drude, Paul. Calibration of wave 
meter, 117; resonance method of 
measuring wave length, 216; wave 
meter, 216. 

Duane, velocity of waves on wires, 
68, 69. 

Duddell, W. Law of distance, 129; 
thermo-galvanometer, 129, 154; 

singing arc, 254, 255. 

Dunwoody, H. H. C., carborundum 
detector, 160. 

Duplicity of vibration of coupled cir- 
cuits, 235. 

Dynamometer, high-frequency, 113. 

Earth. Propagation of electric waves 
over, 122, 125. See Ground. 

Edward VII, message from Pres. 
Roosevelt to King, 107. 

Einthoven galvanometer, 141, 263. 

Electric force, related to potential 
gradient, 334. 

Electric waves. Maxwell's theory of, 
5, 36, 38; Hertz’s experiments on, 
5, 43, 50, 51, 66; properties of, 40, 
48; interference of, 45, 46, 47, 48; 
refraction of, 40, 55; velocity of, 
in air, 40, 50, 69; of short wave 
length, 51, 56, 59, 60; polarization 
of, 54; table of, 60; on wires, 62, 
66, 74; velocity of, on wires, 66, 
68, 69, 70, 74; from grounded os- 
cillator, 124. 

Electricity. Theories as to nature of, 
6; and magnetism, 12; elementary 
facts about, 329. 

Electrolytic detector, 201; current- 
voltage characteristic, 203; oscillo- 
graphic study, 205; conclusions re- 
garding, 211. 


Electromagnetic theory of light, 5, 
36, 41. 

Electrometer. Capillary, 142; abso- 
lute, 329. 

Electromotive force. Of condenser, 
25; definition of, 334. 

Electron, mass and charge of, 9. 

Electrostatics, 23, 329. 

Energy. Relation of magnetic field 
to, 21; and e.m.f. of charged con- 
denser, 25. 

Engineering details^ 312. 


Fahie. History, 75, 82; letter to, 158. 

Falling characteristic, 171. 

Farad, 24, 336. 

Faraday, Michael. Electrolysis, S, 
9; current from magnetic field, 16; 
electrostatics, 23; dielectric, 24; 
basis of Maxwell’s theory, 36. 

Feddersen, rota ting-mirror photo- 
graphs, 3. 

Fessenden, R. A. Barretter, 154; 
electrolytic detector, 201, 203; 

high-frequency alternator, 306; 
tower at Brant Rock, 316. 

Field of electric force about oscilla- 
tor, 49, 124. 

Field of magnetic force, 13, 50. 

Fizeau, velocity of electric propaga- 
tion, 62. 

Fleming, J. A. Dynamometer, 113; 
note on Zen neck’s theory, 125, 
133; oscillation valve, 212; wave 
meter, 220; method of measuring 
capacity, 224; on directive antenna, 
299. 

Formulas. For current during dis- 
charge, 31; period of discharge, 35; 
for two wave lengths in coupled 
circuits, 236; for period of arc, 
260; for sending capacity, 319; for 
high-frequency resistance, 337 ; for 
calculating capacity, 339; for cal- 
culating inductances, 341. 

Franklin, Benjamin, theory of elec- 
tricity, 7. 

Frequency meter. See Wave meter. 



INDEX 


347 


Light. Electromagnetic theory of , 3, 
41; identity of electric waves and, 
56; table, 60; effect on transmis- 
sion, 133. 

Lindsay, J. B,, signaling through 
water, 76. 

Loadstone, 12. 

Lodge-Muirhead-Robinson, coherer, 
144. 

Lodge, Sir Oliver. Resonance ex- 
periment, 42, 215; use of coherer, 
81; patent of resonant circuits, 93; 
system of wireless telegraphy, 97. 

Loops of potential and current, 45, 
46, 47, 48, 111. 

Lyman, Theodore, ultra-violet light, 
60. 

Macdonald, wave length of oscil- 
lator, 116. 

Madelung, E., on magnetic detec- 
tor, 151. 

Magnet, 12. 

Magnetic detector, 145, 146, 147, 
151, 153. 

Magnetic; Field, 13, 14, 15, 16; about 
a Hertz oscillator, 50. 

Magnetism, relation between elec- 
tricity and, 12. 

Magnetization by condenser dis- 
charge, 2. 

Mandelstam, phase-difference exci- 
tation, 298. 

Map of stations, 326. 

Marconi, ( lugiielmo, 80; first patent, 
S3; 1890 apparatus, 83; grounded 
circuits, 85; coherer, 85; deco- 
hering devices, 85; “ claims , *’ 90; 
achievements between 1896 and 
1898, 91 ; coupled circuits, 103; 
duplex apparatus, 105; achieve- 
ments in 1901-1902, 106; effect of 
daylight, 133; company, 139; mag- 
netic detector, 146; reflectors, 296: 
directive antenna, 29S. 

Maurain, C.,“ suppression of hys- 
teresis, 149. 

Maxwell, James Clerk, electro-mag- 

netic theory, 5, 36, 41, 


Medium, influence of intervening, 23. 

Mercury-arc oscillator, 307. 

Method of wireless telephony, 305. 

Microfarad, 24. 

Microphone. As detector, 15S; ap- 
plied to wireless telephony, 309. 

Mirrors, cylindrical metallic, 51. 

Molybdenite, 161, 177, 178; oscillo- 
grams of, 1S6; thermo-electric prop- 
erties, 189. 

Monarch, repair ship, 129, 131. 

Morse, S. F. B., telegraphy by con- 
duction through water, 75. 

Mounting for molybdenite detector, 
179. 

Muirhead, coherer, 144. 

Nasmyth, G. W., period of arc, 260. 

National Electric Signaling Co., 139. 

Navy, U. S., Stations on Atlantic 
Coast, 326. 

Nodes, 45, 46, 47, 48, 111. 

Northrup, E. F., dynamometer, 113. 

Oersted, H. C., relation of electricity 
to magnetism, 13, 14. 

Oil, castor, condensers submerged in, 
317. 

Oil, vaseline, spark in, 57. 

Optics of electric oscillations. Rigid, 
56. 

Oscillation. Spark-photograph of, 3; 
period of, 35; number, s7 ; nature 
of, 108; of coupled systems, 228; 
photograph of double, 248; har- 
monic, 279. 

Oscillator. Hertz, 44; field about, 
49; rectilinear, 51; for short wavo, 
59; Marconi, S3; wave-length of, 
116; mercury-arc, 307. 

Oscillatory discharge. Siv Conden- 
ser. 

Oscillographic study, of crystal recti- 
fiers, 181; of electrolytic de- 
tector, 205, 208; of pendulum 
motion, 233. 

Paalzow, waves on wires, 72, 73; 
bolometer, 154. 

Panama, U. S. Stations in, 326. 



INDEX 


349 


Resonator. Hertz’s circular, 44; rec- 
tilinear, 51; Rights, 56; with 
thermal junction, 59. 

ltighi, Augusto, apparatus, 56. 

Rising characteristic, 171. 

Robinson, coherer, 144, 

Robison, S. S. Manual of Wireless 
Telegraphy, 219. 

Roentgen rays make gases conduc- 
tive, 9. 

Rogers, telegraphy through water, 76. 

Roosevelt, President, message, 107. 

Rubens. Waves on wires, 72, 73; 
telegraphy by water conduction, 
77; bolometer, 154. 

Rutherford, E., magnetic detector, 
145. 

Sarasiti. Repetition of Hertz’s ex- 
periments, 68; spark in oil, 57. 

Saunders, velocity of waves on wires, 
OS. 

Schloemilch, electrolytic detector, 201, 
203. 

Sehmidt-Wilkes telephone receiver, 
sensitiveness of, 140. 

Schumann, V., ultra-violet, light, 00. 

Seawater, propagation of electric 
waves over, 125, 131. 

Sending station. Tuning of, 243; 
construction of, 312. 

Shadows, cast, by metallic screens, 52. 

Shoemaker, electrolytic detector, 203. 

Shunt capacity, tuning by, 273. 

Shunted telephone, used with detec- 
tor, 135. 

Silicon, 161; steel, 19S. 

Silver, removal of, 324. 

Simon, H. Tli., talking an;, 254. 

Singing an;, 253, 260, 264. 

Singing spark, 253. 

Skin effect, 70, 337. 

Soil, propagation over, 126, 131. 

Spark. In oil, 57, 268; potential, 29, 

56; photographs, 3, 248; quenched, 

253, 266, 269; singing, 253. 

Spectrum of electric waves, 60. 

Station, diagram of circuits of com- 
plete, 322. 


Stationary waves, 48; on wires, 74. 
Steam, arc in, 259 
Steel-carbon detector, 158, 198. 

St. John, waves on wires, 73, 74. 

St. Johns, Newfoundland, 106. 
Strecker, telegraphy by water con- 
duction, 77. 

Submarine telephony, limit to, 65. 
Sunset, effect of, 134, 136. 

Sun’s rays, ionization by, 138. 
Surface travel, 69. 

Sympathetic pendulums, 232. 
Syntonic circuits. See Resonance. 

Table. Of wave lengths, 60; of di- 
electric constants, 341; of units, 336. 
Talking arc, 254. 

Taylor, J. E., law of distance, 129. 
Telef unken Co. Arcs in series, 259; 

quenched spark, 267. 

Telegraphy, by wires, 63. 

Telephone receiver, sensitiveness of, 
140. 

Telephony. Line, 65; wireless, 265, 
305. 

Tellurium detector, 160, 198. 

Tesla coil, 93. 

Thermal detectors, 59, 153, 154. 
Thermal junction, use in receiver, 59, 
154. 

Thermo electric, 177, ISO, 196. 
Thomson, Elihu. Transformer, 93; 
dynamometer, 113; continuous 
spark, 253; singing spark, 265. 
Thomson, J. J., electricity and mat- 
ter, 7, 9, 10. 

Thomson, Sir Win. Proof of oscilla- 
tory discharge, 3; criterion, 30; 
period of oscillation, 35; waves on 
wires, 63; absolute electrometer, 
329. 

Torsion balance, 329. 

Tosi, directive wireless telegraphy, 
302. 

Transformer. High-frequency, 93, 95; 

charging, 320; receiving, 323. 
Trowbridge, John, 68, 69, 76. 

Tube. Geissler, 70, 216; cathode, 151. 
Tuning. See Resonance.